More and more young people are being drawn into the world of violent extremism, a senior police officer has warned after a young neo-Nazi caught in an undercover MI5 sting was convicted of planning a mass gun attack.

Alfie Coleman, a former supermarket worker from Great Notley in Essex, compiled a hate-list of colleagues and customers he branded with racial slurs or as “race traitors”. He wrote a “manifesto” in a diary and identified potential targets, including the “lord mayor of London” and a mosque.

The 21-year-old was found guilty of preparing for terrorist acts on Thursday after an Old Bailey retrial. From the age of 14, Coleman had begun to trawl the internet for extreme rightwing material, including a neo-Nazi text he downloaded on his iPad.

He was caught after undercover officers from MI5 engaged with him in encrypted chat as he sought to buy weapons.

The case prompted DCS Helen Flanagan to warn parents to be vigilant, saying that “horrific” terrorist manifestos and other extreme material were just “one click” away.

“Unfortunately, we are seeing younger and younger individuals getting radicalised online,” she said. “Now one in five people that we deal with in counter-terrorism is a child.”

Police were seeing more younger referrals to Prevent, the multi-agency programme that aims to stop individuals becoming terrorists. “It is a concern for us around young people getting caught into terrorism through the online influence.

“Alfie was 14 when he first started to look at content online and we had concerns about his behaviour. I think where people are living their lives online they’re getting exposed, and there is an awful lot of horrific material online that is influencing young people. So, clearly, we are keen to intervene at the earliest opportunity to prevent that ideation and radicalisation happening and turning into a real-world threat.”

Concerns about Coleman were heightened when, in the summer of 2023, he became increasingly active on online extreme rightwing groups. In early September, he arranged to buy a Škorpion automatic weapon, an AK47 rifle and bullets in France after having identified a mosque as his target – but quickly abandoned the plan.

Instead, MI5’s “highly sophisticated operation” culminated in a Morrisons car park in Stratford, in east London, on the morning of 29 September 2023.

That day, Coleman – then 19 – had arranged with an undercover officer to buy a Makarov pistol, five magazines and 200 rounds of ammunition. Jurors saw dramatic video footage of Coleman dropping £3,500 in a Land Rover Discovery and picking up a holdall containing the handgun and ammunition from the boot.

Before he had gone 30 metres, Coleman, who was carrying his Tesco employee card, was confronted by armed counter-terrorism police and forced to the ground.

A search of the home he shared with his parents and sibling revealed the extent of Coleman’s murderous ideology, including idolising Thomas Mair, the extremist who killed the MP Jo Cox.

Police found £2,500 in savings and a device to detect bugs and secret cameras in his bedside drawer, a rock with a swastika on a table, a black sun flag – associated with neo-nazism – on the wall, and various extreme rightwing books.

Police also seized a collection of knives from his bedside drawer and a small stone axe, an air rifle and a flyer about target shooting on top of his chest of drawers.

An analysis of his electronic devices revealed that in July 2021, Coleman had emailed the far-right white supremacist organisation Patriotic Alternative saying he “would like to start participating in activism”.

He went on to write down plans for potential terrorist attacks, such as hijacking a plane and targeting the home of the ceremonial lord mayor of London – whose address he’d mistaken for that of the elected mayor of London, Sadiq Khan. The plans involved putting explosives in a cash machine, as well as the use of knives and crossbows, the court was told.

He was “seething with hatred” as he created a list of people at work who had “upset” him in September 2022, the prosecutor Nicholas De La Poer KC said. Among those he singled out was a white female co-worker who was married to a man of mixed Indian and Seychellois heritage.

The defendant’s “manifesto” drew inspiration from several extremist mass killers whom he regarded as “warriors”.

Giving evidence, Coleman described being lonely and suffering with his mental health during the Covid-19 lockdowns.

He had admitted attempting to possess both a firearm and ammunition but denied he was preparing for a terrorist attack. He had pleaded guilty to possessing 10 documents with information likely to be useful to terrorists such as texts on weaponry and bomb-making instructions.

After the verdict, Coleman was remanded into custody to be sentenced on 8 July.