A veteran republican who helped found the Provisional IRA has said Gerry Adams was a member of the organisation and described his denials as "a lie".
Des Long has given an interview to BBC Spotlight as part of its series on the Troubles.
He claimed he attended meetings of the IRA's ruling executive with Mr Adams, who was there as "chairman of the army council".
Mr Adams has always maintained he was never in the IRA.
Mr Long's allegation is made in part two of Spotlight on The Troubles: A Secret History, which charts the IRA's campaign of violence through the 1970s.
He said: "The chairman of the army council would turn up to the executive meetings. We were meeting at least every four months."
When asked who the chairman was, he replied: "Gerry Adams."
It was then put to Mr Long that the former Sinn Féin president has repeatedly denied ever being in the IRA.
He replied: "I sat opposite him at meetings. This lie he comes out with, he was never in the IRA, that's a lie.
"I'll probably get shot for saying it, but I'm saying it."
Mr Long fell out with Mr Adams in 1986 as part of a wider split within republicanism.
The programme also uncovers secret army intelligence documents which state Mr Adams was a leading figure in the IRA.
Lord Ramsbotham, a former army commander in Northern Ireland, said Mr Adams was "a strategic planner of the highest order".
Mr Adams, who is a TD for Louth, last denied he was in the IRA when he gave evidence at the Ballymurphy Inquest in May.
He declined to take part in the Spotlight programme.
Sinn Fein's Conor Murphy dismissed the comments by Des Long, who he called a "bitter opponent of Gerry Adams for 35 years (who) would have regarded (Adams) as a traitor".
"The allegations have been made over the years and have been consistently denied," Mr Murphy told BBC Radio Ulster's Good Morning Ulster programme.
"You have to look at the people who are making the allegations to see what their motivation is."
Episode Two of Spotlight on the Troubles: A Secret History will be shown Tuesday, 17 September at 21:00 BST on BBC One Northern Ireland and BBC Four. Episode One can be viewed now on the BBC iPlayer.