After meeting an Irish judge investigating the 1974 Dublin and Monaghan bombings, Northern Ireland Secretary John Reid confirmed that police and forensic documents had already been handed over to the independent commission.
He said: "I will do all I can to see we are as helpful as possible."
Families of the victims had persistently complained that Britain had failed to comply with the commission, headed by Mr Justice Henry Barron, with a "slow" response to a request to hand over security documents about the atrocities.
The judge is investigating claims of British collusion with loyalists who bombed Dublin and Monaghan in May 1974, which killed 33 people.
Dr Reid said tonight an "extensive trawl" of Whitehall files had been undertaken in preparation for their disclosure to the commission.
After meeting Mr Justice Barron at the Northern Ireland Office in London, Dr Reid said: "I assured him of the seriousness with which the Government took requests for information received from the commission.
"That is why the UK authorities, specifically the Northern Ireland Forensic Science Service and the RUC (now PSNI), had already supplied large amounts of material at the request of the commission."
The issue was raised by the Taoiseach Mr Ahern with his British counterpart Tony Blair at a meeting of the British Irish Council at the end of last year.
In Dublin earlier today Mr Ahern said it was essential that the "substantial details" contained in the Whitehall files were disclosed to the commission.
He said: "Mr Justice Barron, for the better part of a year now, has made it clear that he requests the co-operation of the British authorities - particularly the Northern section and the security divisions.
"The Irish Government and Mr Justice Barron believe there must be substantial details in the files."
He added: "It is imperative that the Secretary of State, at this fairly long drawn out attempt to have a meeting, co-operates fully with Justice Barron.
Pressure group Justice for the Forgotten tonight attacked Dr Reid's comments as "absolutely negative".
The group, made up of bereaved relatives and survivors of the outrage, said Dr Reid had been "vague and non-committal" in his statement following the meeting.
Group secretary Margaret Unwin said: "He is not responding to the request for disclosure at all - he is saying they will be helpful, that is all.
"There is nothing to say when the documents will be handed over. In fact he doesn't actually say any files will be disclosed."
She added: "It is absolutely terrible and totally negative. We have waited 11 months since the request for the documents and all we get are a few vague and non-committal words."
The group would discuss the meeting in greater detail with Mr Justice Barron on his return to Dublin, she said.
PA