The author of a book that was one of the reasons the Smithwick Tribunal was established is refusing to attend to give evidence.
Toby Harnden, who is now the US editor of the Daily Mail and the US executive editor of the Mail Online, had been due to give evidence over two days this week.
He was the author of ''Bandit Country - the IRA and South Armagh'' in which the claims of garda collusion in the murders of two senior RUC officers were highlighted.
The Smithwick Tribunal is investigating claims that the IRA murders of Chief Superintendent Harry Breen and Supt Bob Buchanan were assisted by an informant within Dundalk Garda Station.
They died in an ambush just minutes after leaving a meeting in Dundalk in March 1989.
Counsel for the tribunal, Fintan Valentine told Judge Smithwick today that Mr Harnden would not now be giving evidence and he said he was unavailable.
The decision, according to Mr Valentine, appeared to have been decided after consultations between Mr Harnden and his new employers, Associated Newspapers.
Jim O'Callaghan, counsel for former Detective Sergeant Owen Corrigan, who is one of three gardaí being investigated by the tribunal, said Judge Smithwick was entitled to take an inference from the non-appearance of this witness.
Judge Smithwick said the dates had been chosen by Mr Harnden after the tribunal had originally suggested giving evidence before Christmas.
The tribunal had been given very short notice of his change of mind, but the judge said he would prefer not to say very much at this stage.
He said they would try to get Mr Harden to give evidence at a later date.
The Tribunal also heard from retired Assistant RUC Chief Constable Raymond White that Freddie Scappaticci was a member of the IRA's internal security unit known as the "nutting squad".
However, he refused to confirm that Mr Scappaticci was the British agent known as "Stakeknife".
Mr Scappaticci has always denied the claims.