Sinn Féin's Gerry Kelly has said the party has been told by the family of Freddie Scappaticci, the man dubbed the British Intelligence agent 'Stakeknife', that he has not left the country.
Mr Kelly said Sinn Féin advised the family that they should consult a solicitor and make a public statement about the allegations.
Meanwhile, the Taoiseach has said the Government will raise the issue with the British government next week.
Bertie Ahern said these were serious allegations which raised 'fundamental issues', and added to an already long list of accusations about British intelligence, including the Stevens inquiry and the murder of Belfast solicitor, Pat Finucane.
Pointing out that there had been a lot of 'extraordinary things' over the past 25 years, he said that it gets more worrying as we get to the 'endgame'.
He added that the British organisation known as the Force Research Unit seemed to be involved in many of the allegations.
Mr Ahern said the allegations had to be dealt with as a security issue.
He said the cases did not help any side, and that 'we' had to be careful to deal with them, but at the same time not allow them to damage the future.