The Labour MP for Brent East appeared as a witness for Jimmy Smyth, one of 38 republicans to escape from the Maze in 1983, despite reports of rumblings among senior Labour politicians, who fear he will damage the party by being seen to endorse IRA terrorism.
But Mr Livingstone made clear to a US district judge that he believed Smyth was not guilty of attempting to murder a prison officer - the crime for which he was sentenced to 20 years in prison by a Diplock court in 1978 - and was not a member of the IRA.
In a crucial test case, which threatens to cause considerable embarrassment to Britain, Smyth argues that he should not be sent back to Northern Ireland because, as a convicted republican, he would be at risk of persecution and possible death. Clause 3(a) in the amended 1986 US-UK extradition treaty exempts anyone who runs the risk of being victimised or imprisoned because of their race, religion or politics.
Yesterday Mr Livingstone added his voice to that of an earlier witness - the former MP and civil rights leader Bernadette Devlin McAliskey - by saying there was 'overwhelming circumstantial evidence' that some elements of the RUC, the British security forces, and loyalist paramilitaries, have acted in collusion to carry out assassinations against republicans.
Under examination by the defence lawyer Karen Snell, Mr Livingstone testified that British politicians privately concede there is an unofficial shoot-to-kill policy in Northern Ireland. 'Any British Member of Parliament will be willing to say off the record, 'yes there had been an unofficial shoot-to-kill policy',' he said.
Asked if he thought it was safe for Smyth to be returned to Britain, he said that no one identified as pro-republican is safe in Northern Ireland. Although there was no risk from the vast majority of the security forces, individuals took the law into their own hands to kill, torture and intimidate.
At an earlier hearing, the district judge, Barbara Caulfield, passed an order requiring the British government to prove its security forces do not subject Irish nationalists, accused of offences, to retaliatory harm, or even death. She did so after Britain refused to allow her access to top-secret official reports on the conduct of security forces in Northern Ireland, including the Stalker-Sampson investigation.
Mr Livingstone told her that there were 'rogue elements' in MI5, the RUC and the British Army that shoot to kill. 'They tend to get away with it,' he said, 'Rather than expose it, the Government tends to cover it up.'
He also told Judge Caulfield that he had concluded that Smyth was innocent after reviewing her assessment of the Irishman's original conviction, adding - to laughter from the several dozen Irish supporters of Smyth - that he was satisfied she was not 'an IRA stooge or British Army lackey'.
Last night, Kevin McNamara, Labour's Northern Ireland spokesman, moved to distance the party from Mr Livingstone. 'The Labour Party supports the extradition of escaped prisoners to Britain. What Mr Livingstone does, he chooses to do as an individual.'
Smyth was arrested last year for passport offences, after spending eight years living in the United States. Two other Maze escapees are awaiting extradition.