The British security services were involved in formulating the controversial Legacy Act, which offered an amnesty to soldiers and paramilitiaries despite MI5’s role in many killings during the Northern Ireland Troubles, it can be revealed.

The presence of policing and state agency figures among a secret policymaking group involved in devising the act – a fact established through an investigation by Belfast-based newsletter the Detail and shared with the Guardian – has angered victims’ groups already critical of the legislation.

The 2023 act’s conditional immunity, which the current government removed after a vote in January, was opposed by all political parties in Northern Ireland, albeit sometimes for different reasons.

Daniel Holder, from the Committee on the Administration of Justice (CAJ), a Belfast-based NGO, fought an eight-month battle to have documents relating to the legacy senior working group released after his freedom of information requests were initially rebuffed.

Holder, who shared the documents marked “official secret” with the Detail, said: “It is only now that some detail of the group has emerged. The legacy investigations senior working group was set up behind closed doors to assist the development of what became Boris Johnson’s government’s notorious Legacy Act.

“Despite the legal duties to ensure effective and independent investigations into legacy cases, which clearly include those involving the security forces, the group itself tasked with advising on how the policy should be developed heavily involves policing and security figures.”

The revelations that security services were involved in the act’s formulation lend weight to long-held public concerns that the security and policing services were behind the Legacy Act and the Independent Commission for Reconciliation (ICRIR), the body it set up to investigate cases. The previous attempt to address legacy issues, the Historical Enquiries Team, was folded in 2014 after it was found that it had failed to properly investigate state killings.

Holder said the act that emerged from the meetings “led to the shutting down on the 1 May 2024 of hundreds of cases under the then existing legacy mechanisms and their disastrous replacement with the ICRIR, which more than two years on is yet to complete a single case”.

Members of the group, established in 2020, included the former Police Service of Northern Ireland (PSNI) chief constable George Hamilton, and Madeleine Alessandri, who was the UK deputy national security adviser until 2020, before being appointed permanent secretary for the Northern Ireland Office. Another attender, Chloe Squires, was the director of national security at the Home Office at the time of the meeting. The rest of the attenders worked for, or had links to, the state, chiefly its policing and security apparatus.

Minutes taken from a meeting recorded Hamilton as saying: “Families will welcome information recovery. It is the small vocal minority that will present the legal challenges and we should be ready for that but they do not speak for the silent majority who just want to move on.”

In response, Mark Thompson, of Relatives for Justice, said: “I wouldn’t describe the 1,100 bereaved relatives [reflecting the number of civil cases lodged by bereaved families when the Legacy Act was introduced] of murder victims including victims of torture who want answers as a ‘small and vocal minority’.”

He added: “It is not lost on all these families that the very people who made these comments represent the very same state agencies that would be subject to robust independent investigations with full accountability if we were to have such a process.”

Another released document presented a list of suggested talking points for the chair of the secret group. It underlined the government’s aim to conclude the investigations of the 3,500 deaths that occurred during the Troubles “within two years”, a timeframe it acknowledged as “ambitious”. It said: “A key component of the policy package is the intention to introduce a legal bar on further criminal investigations or prosecutions as a way of providing certainty to veterans and victims and ‘unlocking’ further information recovery opportunities.”

The documents show that the legacy senior working group met on 19 June and 20 July 2020. The files indicated that there was a third meeting scheduled but no further information has been disclosed about it and a government source indicated that it did not occur.

Holder said the “very existence of the group only seems to have been revealed incidentally when the then PSNI chief constable, Simon Byrne, to his credit, told a Westminster committee that the PSNI had declined to join the group in order to remain impartial”.

A Northern Ireland Office spokesperson said: “This relates to a group that was established to discuss issues relating to the development of the previous government’s Legacy Act, which this government is repealing and replacing.”

Hamilton said: “My sight of the minutes is almost six years after the event. In light of this, I cannot confirm that any of the comments attributed to me were accurately recorded …

“My agreement to be part of the working group was to ensure the lessons of previous attempts at dealing with legacy investigations were understood and to ensure learnings were not lost in the drafting of any new proposals. Most importantly, I emphasised the importance of having victims and families at the centre of any investigation, review or information recovery process.”

He also said that excerpts of what he said were selective and had been taken out of context.