The PSNI refused to hand over intelligence files about alleged garda collusion with the IRA before its most senior anti-terrorist officer gave public testimony at the Smithwick Tribunal last October, it has been learnt.
This sparked the attack on PSNI Assistant Chief Constable Drew Harris by counsel representing Garda Commissioner Martin Callinan at Friday's final day of hearings at the tribunal in Dublin into alleged garda collusion in the murders of two senior RUC officers in 1989.
Now, senior sources say, Government intervention will be needed to heal a "massive" rift between the Garda and the PSNI over the allegations made by Harris at the tribunal, which were vigorously rebuffed by Callinan's counsel as "nonsense on stilts". Harris heads the Criminal Operations branch of the PSNI and is in overall charge of serious organised crime and terrorist investigations.
The dismissal – in public – of the testimony of such a senior officer by a Garda Commissioner in such trenchant terms has never happened before.
Harris's testimony last year sparked a major row which was kept secret at the time. Gardai were informed beforehand that he was to allege garda-IRA collusion but a request to see the "evidence" for this before it was made public, so it could be investigated, was refused.
Both governments have consistently praised the co-operation between the forces in anti-terrorist work, but Callinan's rebuff to the PSNI revealed the stark rift at the most senior levels.
Garda sources believe the PSNI was prevented from handing over the alleged intelligence at the behest of the British security service MI5.
In his evidence last October, Harris, who has control of the PSNI Special Branch which works closely with MI5, said: "During 2011, a senior PIRA member confided to an associate their personal fears concerning the tribunal, particularly that An Garda Siochana personnel that were previously under PIRA's control would potentially highlight the level of co-operation previously provided."
At the final day of the tribunal hearings, the Garda Commissioner's counsel, Diarmaid McGuinness, insisted that these allegations were "nonsense upon stilts".
He said Harris had told the tribunal there was no RUC intelligence at the time of the murders that suggested collusion by any member of An Garda Siochana in the ambush.
But 24 years later, he said, the tribunal was faced with a "Niagara of intelligence", with Harris swearing it was all accurate.
The withholding of intelligence by the PSNI, he said, "cast the gravest shadow over the bona fides, the willingness and ability of the PSNI to co-operate with the tribunal".
Mr McGuinness accused the PSNI of failing the families of the late Chief Supt Breen and Supt Buchanan; failing An Garda Siochana by not sharing this intelligence; and ultimately failing the tribunal itself.
Based on the totality of the evidence before the tribunal, there was no evidence of any garda collusion and that was the submission of the Garda Commissioner, he added.
Senior Garda sources yesterday said the rift between the senior levels of the forces was the most serious since relations between the Garda and the then Royal Ulster Constabulary broke down for three years in the early 1980s.
That occurred after the gardai had asked the RUC to arrest a Fermanagh man to prevent him giving evidence in an assault case in Cavan District Court against Martin Nangle, the brother-in-law of the then Minister for Justice Sean Doherty. Gardai falsely accused the witness, James McGovern, of having IRA links. Mr Nangle was acquitted when Mr McGovern failed to appear.
The 'Dowra Affair', as it was known, caused a breakdown in co-operation that was only ended when Margaret Thatcher agreed to give the Republic a say in the running of Northern Ireland and the establishment of the Anglo-Irish Secretariat.
In return, the Garda and RUC set up a hotline communications systems and officers from both sources embarked on closely co-ordinated cross-border operations against the IRA.
Judge Smithwick is to deliver his report later this year into allegations of collusion in the murders of Chief Superintendent Harry Breen and Supt Bob Buchanan after they had left Dundalk garda station on March 20, 1989.
A PSNI spokesman said there would be no comment "about An Garda Siochana's closing submission", adding: "We will leave Judge Smithwick to determine respective roles and responsibilities."