The family of murdered Belfast solicitor Pat Finucane regards as significant the offer by the British intelligence service to make available its files to a future inquiry.
The murder of Mr Finucane, shot dead in front of his family at their home in 1989 by loyalists, remains among the most contentious of the Troubles because of allegations of collusion with the police.
However the Finucane family and the British government remain at odds over the nature of the type of investigation that should be mounted.
The British government is insisting that an inquiry can only be held under the 2005 Inquiries Act which empowers ministers to withhold key findings. The offer by MI5 to submit its files is strictly dependent on an inquiry being held under this legislation.
The Finucane family, backed by nationalist political representatives, says only an independent, international inquiry is acceptable.
Peter Madden, the late Pat Finucane's legal partner, has said the MI5 offer which was contained in US diplomatic communications from former special envoy Mitchell Reiss and made public by WikiLeaks, is significant and will be considered by the family.
Northern Secretary Owen Paterson, who has met Mr Finucane's widow Geraldine, is considering what to do and is expected to make an announcement in the New Year.
Both Sinn Féin and the SDLP insisted last night that the holding of an inquiry under the new legislation, as demanded by MI5 and the British government, remains unacceptable.
Sinn Féin junior minister Gerry Kelly has claimed the Inquiries Act was initiated at British intelligence services' insistence to prevent the truth.
This revelation adds further significant weight to the Finucane family campaign for a fully independent international inquiry into the murder, Mr Kelly said.
"The British State has to date never acknowledged holding files and information into the murder of Pat Finucane," he said. "Instead they have hidden behind a long standing campaign of concealment and cover-up."
He added: "The revelations that MI5 are holding files directly related to the murder of Pat Finucane and that the Inquiries Act 2005 was initiated at their insistence to prevent the truth from being revealed adds significant further weight to the family demand for a fully independent international inquiry into the murder. That is what the British government agreed and that is what the British government must deliver."
SDLP leader Margaret Ritchie agreed that the MI5 offer, as signalled by Mitchell Reiss is "potentially very significant".
She said the revelation that the files existed begged a series of questions of MI5.
"Why do they have them and what is in them? And secondly there are further questions to be asked why MI5 are in possession of intelligence that perhaps the PSNI are not," she asked.
"The SDLP will always remain vigilant about any partial disclosure or attempts to suppress information or clean files prior to publication as this is a distinct possibility under current legislation.
"The SDLP's priority is for the Finucane family to get the truth and justice they seek and deserve. The only way to achieve this is for full disclosure of any relevant information to the case.
She concluded: "A censored inquiry will not be accepted."