Northern Ireland's Deputy First Minister Martin McGuinness was involved in the IRA sanctioning an operation to abduct, torture and murder two of the most senior RUC officers killed in the Troubles, an inquiry has been told.
British intelligence officer Ian Hurst - also known as Martin Ingram - claimed to have inside knowledge linking the Sinn Fein chief to an order for the 1989 border ambush of Chief Superintendent Harry Breen and Superintendent Bob Buchanan.
Mr McGuinness said the former soldier has no credibility.
"By his own admission he is part of a British security apparatus which played a very negative and malign role in the conflict, including widespread involvement in collusion," his spokesman said. "His submission to the tribunal needs to be seen and judged in that context."
The Smithwick Tribunal into IRA-Garda collusion in the Republic was told Mr McGuinness was in the IRA's northern command and "involved" when terror chiefs sanctioned the double murders on their way home from a cross-border policing briefing. It heard about between 30 and 60 people would have been involved in the murder ambush.
Ingram, as he was known outside intelligence circles, told the inquiry that intelligence on the Buchanan and Breen atrocity came from the high-level double agent in the IRA known as Stakeknife, allegedly notorious Belfast republican Freddie Scappaticci.
"Basically I mean cipher 82 (Stakeknife's handler) had information like it was authorised at northern command and Mr McGuinness was involved as OC," the inquiry was told.
A spokesman for Mr McGuinness, a self-confessed Provisional IRA commander in Londonderry in the early 1970s, told the Press Association that Ingram was a dubious character. He also claimed the tribunal has questioned other British intelligence evidence.
Ingram was a member of the British Army's covert Force Research Unit (FRU) in Northern Ireland which itself has faced damning allegations of collusion, for gun running with Loyalist paramilitaries and more than a dozen murders.
He was cleared to give evidence in secret by the Ministry of Defence. The claim against Mr McGuinness was part of redacted transcripts read into the record.