The Smithwick Tribunal, which is investigating claims of garda collusion with members of the Provisional IRA in the murder of two RUC officers, has broken off contact with former paramilitaries after they refused to give oral evidence.

In his third interim report, Judge Peter Smithwick said that despite "extensive safeguards" to protect their identity and security, none of former members of PIRA were willing to give oral evidence.

He said he was "very disappointed" at what has happened.

The Tribunal is investigating allegations that members of the gardaí passed information to the Provisional IRA which allowed them ambush RUC Chief Supt Harry Breen and Supt Bob Buchanan.

They died minutes after leaving a meeting in Dundalk Garda Station in March 1989.

Three retired garda sergeants; Owen Corrigan, Leo Colton and Finbarr Hickey have all denied allegations they were an IRA mole in Dundalk Garda Station.

In his interim report, Judge Smithwick said the Tribunal's legal team met with three former paramilitaries in April 2011, one of whom had direct knowledge of the murder of the two RUC officers.

Since then the Tribunal has been trying to secure their co-operation to get one of them to give evidence.

Last November the Tribunal sought to have a further private meeting with them to clarify certain issues.

A scheduled meeting was later cancelled and six days ago they instead provided the Tribunal with a written statement but would not be giving evidence.

Judge Smithwick said there will be no further meetings with the former PIRA members.

The original meetings between the Tribunal and the paramilitaries had been facilitated by Sinn Féin.

The details of the Tribunal's dealings with the former paramilitaries - including their account of the operation to kill the RUC officers - will be made public tomorrow.

This will be the first public sitting of the Tribunal in several months.

It has been adjourned as one of the key witnesses, former Det Garda Owen Corrigan, had to undergo a medical operation.

Judge Smithwick said he expects the retired officer to be able to give evidence in the spring.

Contacts are still continuing with the Police Service of Northern Ireland to obtain a summary of intelligence reports which the Tribunal believe are important.