A nihilistic internet suicide forum implicated in over 160 UK deaths has been fined £950,000 by the online regulator in its latest attempt to shut it down.

Ofcom said the US-based website remained accessible in the UK despite over a year of warnings. Online safety campaigners have accused the regulator of taking an “interminable” amount of time to act.

The Samaritans, mental health campaigners and the Molly Rose Foundation have repeatedly raised concerns about the site, which has remained accessible despite it being cited in multiple coroners’ reports on the deaths of UK citizens.

The Molly Rose Foundation was set up in the memory of Molly Russell, a 14-year-old who took her own life after descending into a vortex of negative online content, including about suicide. Its chief executive, Andy Burrows, welcomed the fine and a separate move that could block UK internet access to the site, but said it was “appalling that it has been left to bereaved families and campaign groups to press Ofcom into action”.

Ofcom has been trying to get the site to obey British laws criminalising intentionally encouraging or assisting suicide since last spring. It had some success with the site being blocked last July and then a mirror site being taken down in November. But it is now taking action because the site can be “used by people in the UK, including without a VPN, and presents a material risk of significant harm”.

The fine is being levied under the Online Safety Act which also allows Ofcom to seek a court for an order requiring internet service providers to block UK access to the site. The regulator is preparing an application to have the site’s connections effectively cut “if our concerns are not fully addressed and there continues to be an ongoing breach”.

It accused the provider of “serious and deliberate contraventions” and said the fine reflected “the risk of fatal harm to people in the UK posed by the content present on the service”.

On Wednesday, the forum was unavailable but its operator posted a page that said it was advocating for “the right to access lawful information without government overreach” and quoted Mark Twain: “Censorship is telling a man he can’t have a steak just because a baby can’t chew it.”

The Molly Rose Foundation and Together with Families and Survivors to Prevent Online Suicide Harms said coroners had warned the UK government 65 times about risks of further deaths from the forum “and a substance it promotes, glorifies and instructs for use as a suicide method”.

Adele Zeynap Walton, the sister of Aimee Walton who took her life after accessing suicide forums, said the wait for action had been agonising. “While we’ve waited further lives have been lost and we’ve had to fight every step,” she said, speaking on behalf of Families and Survivors to Prevent Online Suicide Harms. “We feel let down by the process and Ofcom’s slow response to this threat to life.”

Burrows said: “Molly Rose Foundation submitted detailed evidence which showed scores of vulnerable young people remained at risk while Ofcom’s investigation dragged on … There are real questions about why it has taken so long for the regulator to act against a forum linked to at least 164 UK deaths.”

Ofcom said it had “engaged extensively” with the forum provider and that last summer the forum had geoblocked mirror sites in the UK and later removed information on a landing page promoting ways to circumvent the block.

“We share the urgency about the extreme harms that sites such as this can cause, and understand the anger felt towards them by those who have been so personally affected,” a spokesperson said. “It is vital that we ensure our enforcement action is thorough, and this can take time, as is the case for any enforcement agency.”

“Lucas was 16. Vlad 17. Aimee 21. Grace, Hannah and Tom 22. Immy 25. Adam 28 and Claire 41,” the campaign groups said in a report last year. “They were drawn into a dark world that was allowed to exist online and continues to exist through the use of a VPN. We believe our loved ones suffered coercion, grooming, instruction on how to end their lives. Most accessed a poison that was allowed to cross borders or was readily available domestically.”

Suzanne Cater, the director of enforcement at Ofcom, said the forum had “caused unimaginable pain and suffering … and no punishment can undo that harm”.

“The provider of this forum knows it’s used to share illegal content encouraging and assisting suicide on their site,” she said. “While they’ve responded to our enforcement action by making some changes to the accessibility of their service in the UK, this is not good enough and the changes they’ve made were not consistently applied or effective to reduce the risk of harm. Given the ongoing risk of harm, we are using all powers available to us to protect the public.”