For the first 20 years of her life, Emily had what she thought was a “completely normal” relationship with her dad, Mark. “He was an ordinary man,” she says. “A good dad. We were really close.” Then one morning, police officers arrived at her family home to arrest him for sexually abusing her. Emily wasn’t there. “I had just moved out to live with friends and start my first proper job,” she explains, “but the police didn’t know that. They were trying to protect me.” Emily is telling this story two years on, with her mum, Fiona, by her side. They are close, supporting each other during this difficult conversation, finishing each other’s sentences.

When Fiona heard the door go at 7am, she had just got up. “I wasn’t even fully dressed,” she says. “It sounds stupid but I had just got on an exercise bike so I was in a T-shirt and pants. I looked out of the bedroom window and saw eight people on the doorstep. They weren’t in uniform but they looked official. They had lanyards on and a dog with them.

One of the women looked up at me and our eyes connected. She pointed at the door, as if to say ‘Get this now’, and I knew then it wasn’t friendly.”

At the top of the stairs, Fiona saw Mark already at the door. He didn’t seem confused; it was almost as if he knew why they were there. Still, her mind didn’t go to abuse.

The police put Fiona and Mark in separate rooms and searched the house with their dog. As well as looking for devices such as laptops, it quickly became apparent that they were looking for Emily. “They kept asking, ‘Where’s your daughter?’ I immediately thought something bad had happened to her.”

It was only as Mark was led away to the station that a senior female officer finally told Fiona why they were there. “They said to me, ‘Your husband has been sexually abusing your daughter.’”

Through her shock and confusion, Fiona immediately thought it couldn’t be true. “I am so close to Emily. I felt strongly that I would have known.” But the police had horrifying news for Fiona. “They told me he had been caught on a chat forum describing how he had been raping and sexually abusing Emily for years. It was written as a confession. He’d even used her name and talked about where we lived.”

The random stranger Mark thought he had been talking to was an undercover police officer. Now that officer was standing in Fiona’s living room.

“I was sitting shaking with shock. They were staring at me and I felt I was on trial. I could see the officer thought I was naive when I said I couldn’t believe he had abused her. They said it had started when he gave her baths as a little girl and at that I just said, ‘No, he never gave her baths.’ He wasn’t a hands-on dad. I did everything like that. They were saying he had boasted online about abusing her recently at a family event. But I knew that event never happened. It was not matching with reality.”

The police told Fiona they had a photo that Mark had shared in these chats. “I was terrified,” she says. “I thought it might be something explicit. But it was a photo I knew. Emily was young and wearing a new dress, I remembered taking it, her smiling at the camera. That was the absolute worst moment, realising that he had used that photo in these chats.”

Then the police told Fiona she had to call Emily so they could speak to her. Two years on, Fiona still regrets making that call. “I didn’t think to stop them. I didn’t think: wait, you are going to shatter her world and she’s so far away. To this day she can’t cope with calls that come out of the blue.”

Emily was asleep at the house she shared with friends when her phone rang. “It was a video call that woke me up, and I could see my mum in our living room,” Emily says. “She told me the police were there with her, that they had come to arrest my dad for sexually abusing me.”

Emily’s first thought – just like Fiona’s – was that the police had got this completely wrong. “They took over the call and were asking lots of questions like, did I remember him giving me baths when I was little, did he ever touch me? I just kept saying no.”

The police told Emily the same story they had told Fiona, that online, Mark had been sharing graphic descriptions of abusing her. He said he had been doing it for years. The senior officer explained that they had arrived that morning because they believed Emily was in immediate danger. The police then ended the call, telling her she would need to come home and meet them in person in a few days’ time.

Emily was certain her dad had not sexually abused her. But from that moment her entire view of him – of her childhood – began to unravel. Like her mum, she was in a space of complete horror and confusion.

“Later that day I went to a party with friends, then to the supermarket. I pushed a trolley around while the call went round my head. It made me feel as if half of my memories had died or been rewritten,” she says. “They made me doubt everything I knew about my dad and his view of me. I began to think back on every second we had spoken, every outfit I had worn, every hug he had given me.”

Events like these – the early morning knock on the door, the sense of “a grenade going off in your life”, as Fiona puts it – are alarmingly common. A staggering 1,000 people, nearly all of them men, are arrested across England and Wales every month for viewing or sharing child sexual abuse images.

But Emily’s story is different. When she told the police that her dad had never abused her, she was telling the truth. His sex fantasy chats were just that – a fantasy.

There is increasing concern about the policing of pornography and online sexual fantasies. A growing body of evidence suggests legal but extreme pornography mimicking illegal acts is a major driver of the online child abuse crisis. Convicted offenders are warning that porn algorithms are sending them down “escalating pathways” to ever more extreme material.

All images of children being abused are grounds for arrest, even when the men are not physically abusing anyone. Yet Emily’s case turned out to be not so clearcut in the eyes of the police. Were written fantasies about child abuse, shared on a legal site, against the law? This question would lead Emily all the way to parliament to try to toughen the law on sex chat sites.

But that was all to come. On the day the police arrived at Fiona’s home, they assured her Emily’s memories would “come to the surface now we have asked these questions to trigger them”. She was left alone in the house in total shock. Two days later, Emily came home to talk to the police, who brought with them all the messages to show both women. Until then Fiona had been turning over the possibility that the police might be right; that Emily had buried memories from her childhood. “All weekend I was thinking, have I missed something? Am I such a bad mother that I missed him abusing our child?” But when they were reunited in person, she was left with no doubt.

Fiona was kept out of the room while the police spoke to Emily. “She was interviewed by their sexual assault experts. The detective in charge talked her through the messages. They were very graphic. I think they wanted to shock her; they were pushing her a bit to see her reaction.”

The messages were incredibly painful to read but nothing changed Emily’s mind. Mark had been nothing but an ordinary dad to her – distant, not the most involved, but never abusive.

“It felt as if they were waiting for me to remember this trauma, for that specific crime to be uncovered,” Emily says. “I didn’t feel that they ever really believed me. I signed a form to say I hadn’t been abused and I think at that moment they began to lose interest.”

The sexual assault charges against Mark were dropped and changed to the sending by public electronic communication messages of an indecent, obscene or menacing character under the Communications Act 2003. A court date was set and Emily and Fiona expected Mark to plead guilty as he had never denied the horrendous way he had described abusing Emily online. Emily began to prepare a victim impact statement.

“I was incredibly worried about Emily,” Fiona says. “I could see she was struggling. She very quickly began to talk about her dad and ‘Mark’ as two separate people. I could see her dissociating from that relationship.”

Within days of the arrest, Fiona took radical steps to completely reshape the life that Mark had blown to pieces. “I had a job interview a couple of days later and I just went to it in a daze. I barely remember it but I got the job, and at that moment I decided I would move house and start the new job as soon as I could.”

While Fiona was preparing to move, Emily was going down a rabbit hole into the darkest recesses of the internet. She began to read all she could about sex chat sites and was horrified to learn how easy it is to step straight into sexual chats about children. “I couldn’t believe people were on there openly talking about child abuse. My dad had a user name that was an obvious reference to child abuse,” she says. “It isn’t hidden.”

She wanted the police to know Mark hadn’t touched her, but she wanted him to be prosecuted for sharing his child abuse fantasies online. And she wanted to be recognised as a victim, something the police didn’t seem to understand.

But one day in the run-up to the court hearing, Fiona got a text from Mark saying he was not going to plead guilty. “He said, ‘I’ve found a loophole.’ With help from his lawyer, he had found a way to plead not guilty.” Both Fiona and Emily were devastated. “He showed no remorse,” Fiona says.

Just days before the court hearing, the police got in touch. They were dropping the case. “They told us that, after discussions with the Crown Prosecution Service, they didn’t think there was a realistic chance of conviction. The officer I spoke to told me that in the eyes of the law, Emily was not a victim and therefore no crime had been committed. He actually said that in this situation the ‘victim’ was the undercover officer as they were the ones who read the messages.”

Mark walked away with no criminal record or any form of monitoring. He was not placed on the sex offender register and there is nothing he has to disclose to an employer or a partner.

Fiona has seen Mark only once since the case was dropped: when she met him to get his signature on divorce papers. She saw then how happy he was to have escaped prosecution. “He made it clear that he considered it a prudish response, the public disapproval of a private fetish. We were prudes, the police were as well. It might have been embarrassing to have the messages revealed, but it wasn’t anything that should involve the law.”

The women could not believe this was happening. “How can he talk online about me in this way and walk away?” Emily says. She also found it hard to understand how the sites themselves could escape censure. “If there are so many police on there looking for people talking about child abuse, why can’t they shut the sites down?”

Friends often ask how she is so sure he didn’t really abuse her. “I get why people are asking. They are worried about me. And it gives me a chance to educate them, to explain that this happens all the time; it’s not just a random, unusual thing that happened to me.

I’d like to reiterate that he was an ordinary man. A good dad. There aren’t signs and no one should be expected to spot them before it happens. He was just my dad, and I loved him.”

For both Fiona and Emily, there is a feeling that people looking in at their situation might be judging them, questioning why they didn’t spot the signs. This is acute for Fiona. “I have a particular interest in protecting children from abuse and have always been extra vigilant my child’s entire life. That he spoke this way about her knowing my own background … It was more devastating.”

She and Mark had had their ups and downs. “He was controlling of me. I had discovered in the past he had been chatting to women online. We went to therapy to work on our relationship and I thought we were both putting in effort. Just before this happened I had been feeling he had a swagger to him. Now I know it is because he was still getting fulfilled by a secret online life.”

Emily sees how painful this is for her mother. “My mum is such a strong feminist. She was so heartbroken that, having tried her whole life to protect me from male abuse, this would happen.”

It was in part this feminist rage that propelled Emily to challenge the CPS’s decision to drop the case. She first wrote to her MP, who invited her to his office where she delivered a PowerPoint presentation convincing him this was an issue that went far beyond one family’s trauma. He secured her a meeting with the minister for victims and tackling violence against women and girls, Alex Davies-Jones, who then wrote to the CPS and asked them to explain why the charges had been dropped. In their response, the CPS doubled down on their decision not to press charges.

In a letter seen by the Guardian, they explained that this was because “the prosecution could not prove that the defendant either intended the messages to be indecent or obscene, or that he would be aware of a risk that the messages would be viewed as such by any reasonable member of the public”.

Emily was astonished at the reasoning. “They said that because it was a fantasy and he was talking to someone who wanted to hear it, it wasn’t indecent or obscene. Even though he was talking specifically about abusing me, his daughter. He even named me and gave enough detail that the police found where we lived.”

The letter also explained why a prosecution under the Obscene Publications Act was not considered, saying the meaning of “obscene” must be understood in the legal sense: “having a tendency to depravity or corruption”. “It is this element of the offence which the prosecution could not prove in this case,” the CPS wrote. “The defendant and the recipient of the material were engaged in a private online discussion. The recipient was sending messages back to the defendant which were of a similar nature – and on this basis the prosecution could not prove that the messages – intended to be seen only by the one, like-minded recipient – would have a tendency to deprave or corrupt.”

For Emily this argument felt immoral. But more than that, she wanted to prove it was legally wrong. While researching people who might be interested in her case, she came across Clare McGlynn, a professor of law at Durham University and an expert on legislation around violent and harmful mainstream pornography. McGlynn hears from many victims but when she read Emily’s email it stood out. “I could instantly see the gap in the law that her case highlighted.”

McGlynn has just published a book, Exposed: The Rise of Extreme Porn and How We Fight Back, and by coincidence was researching the links between chat groups and porn that is associated with child abuse when she heard from Emily.

“I had the unpleasant task of going through sites, looking at how many videos there are with themes related to children or incest – or ‘step incest’, which is very popular,” McGlynn says. “I was realising something I hadn’t before: that there is a whole community commenting and sharing stories, interests and links under these videos.”

She could see how the videos created a conversation space for people with similar interests. “So, if you want to talk about how much you like daddy-daughter-themed or stepdad-and-stepdaughter videos – well, here you have a whole community. Of course, the huge danger is that these men can see they aren’t alone. This normalises the whole idea.”

McGlynn argues that the CPS’s interpretation of the law was incorrect. “We have established in case law that you can always further deprave and corrupt. So on that point they are simply wrong. More importantly I think they just did not take this behaviour seriously enough, or understand the danger such men pose. And that is why we need to update the law.” She points to Canada, where it is already illegal to encourage child abuse.

Michael Sheath is another online child abuse expert who advises the police on offender profiling. “We’ve known since the 60s that this is not a defence. There were arguments around whether men in porn shops could be further depraved, and we knew they could be.”

He highlights how men can go from one behaviour to a more serious one – from thinking about porn to thinking about abusive porn, to thinking about abuse in real life. “These sites create an environment where people push boundaries and once you are in that space it’s incredibly seductive. You will learn you aren’t alone; other people will encourage your sexual interest in children.”

With growing understanding of this reaching parliament, McGlynn told Emily she would take her case to her contacts there. Leading the fight for legal reforms in the House of Lords is Baroness Bertin, the Conservative peer who was tasked by the last government with looking at the harms of online pornography.

Working with Labour MP Jess Asato in the Commons, they achieved a recent success, pushing through amendments to the Crime and Policing Act that will ban pornography that “features step-incest or performers role-playing as children”, as Asato said in parliament. In the same speech, she added that “this gateway to paedophilia is swung firmly shut”. Asato told the Guardian that she is very concerned about the failure to prosecute Mark. She believes Emily’s experience shows that legislation needs to be tougher; to respond to the ever changing ways that people are discussing and promoting child abuse online.

McGlynn wants to see “a specific criminal offence to advocate, counsel or glorify child sexual abuse in text”, which would cover discussions in chatrooms and beneath videos on porn sites.

Baroness Bertin, who met Emily as part of her research, says that, as with the planned ban on strangulation in porn, there needs to be a cultural shift around the portrayal of sexual fantasies of incest and child abuse. “We’ve got to reset what’s normal,” she says. “I feel very strongly we want to stop people becoming interested in these extreme topics. When I was researching, there would be millions of views of really violent videos with abusive titles.”

The Guardian contacted the police team that investigated and arrested Mark – a specialist unit in a large regional force. They responded saying that, because all charges had been dropped, they were unable to comment. The CPS said, “Our prosecutors considered a number of possible charges in this case, however, a further detailed review of the file provided by police concluded there was not sufficient evidence to provide a realistic prospect of conviction.”

Emily is waiting to see what will change, but is telling her story in the hope that it will help shift political opinion on the poorly regulated world of online porn and sex chat sites. She also felt vindicated by telling her story to politicians. “It justified for me how I felt when I found out what my father did. I knew it was wrong, despite what the law says.”

She doesn’t want to reveal anything about herself publicly; not her job, her living situation or her passions. Not so that the world can’t identify her, but so her dad knows nothing about her as she moves on into the future. “He knows nothing about me. I’ve even changed my hair so he doesn’t know how I look now.”

In her personal life she is doing well. “I’ve told most people I know. I want friends or colleagues to understand why I act in certain ways. For example, if an older man is sexist to me, that would make me feel incredibly uncomfortable.”

But, despite everything she has been through, she refuses to be cynical about men. “For the first year or so after the arrest, I found it hard hearing dads talking about their kids – just nice, normal things. But I have been able to stay hopeful about life. My mum has always taught me to keep hope. I would never assume men are all the same.”

Fiona, now living hundreds of miles away from the house – and the life – that the police “put a grenade under” early one morning, feels hugely proud of Emily. “She is amazing. She lost everything. She had to question her entire childhood. She lost her grandparents because they took his side. But she has still got her sense of identity. It is incredible.

“One time we were talking and I used the word ‘shame’ about what had happened, about what people would think of us. Emily said, ‘Mum, that’s your shame if you choose to feel it, but I’m not going to have shame.’” Fiona has also been propelled onwards by life, by the need to keep being a parent. “I had to function, to work. I haven’t stopped yet. It’s grief and it comes in waves.”

But Fiona fears that for Mark there is no shame or sanction. When she saw him to get his signature on the documents that would separate their lives for ever, she felt a chill of fear at his lack of remorse. “He should have been charged for what he did, he should have been made to be accountable, and he is pleased that he wasn’t.”

Instead he has been able to move abroad and reinvent himself. “He told me he was dating someone who had kids. It was like he was saying, ‘Look I’ve moved on, someone trusts me.’”

What Mark has lost, however, is his relationship with his child. “He sends a text on birthdays and at Christmas, which Emily dreads, but he doesn’t know anything about her life, and for me that would be a horrendous punishment.”

Emily has the last word on the man who was her dad. “For my own sanity and to preserve my sense of self, I try to separate things out in my mind and keep my childhood memories with him as positive as they once were. That was one person, my dad, and then the person who did this is an entirely different person. I don’t know him at all. He’s the man who wrote disgusting things about his daughter. I have no contact with him, and never will again.”

Names and some details have been changed.