Gerry Adams has won a defamation action against the BBC over a documentary that carried a claim he approved the murder of an MI5 informant in 2006.

A jury at Dublin’s high court on Friday found that the BBC had not acted in good faith or in a fair and reasonable way and awarded the former Sinn Féin leader €100,000 (£84,000) in damages

The verdict came after a high-profile four-week trial that scrutinised Adams’s alleged membership of the IRA and his role during Northern Ireland’s Troubles and peace process.

Lawyers for Adams accused the BBC of a “grievous smear” and “hatchet job”. Lawyers for the broadcaster defended the documentary and said the libel action was a cynical attempt to launder Adams’s reputation.

The former West Belfast MP said a BBC Spotlight documentary and accompanying online article defamed him in 2016 by claiming he had sanctioned the murder of Denis Donaldson, a former Sinn Féin official who was shot dead in County Donegal months after admitting he had for decades been a police and MI5 informant. The claim about Adams was made by an anonymous source known only as “Martin”.

Adams’s lawyers accused the BBC of “reckless journalism” and of making an “unjustified” attack on a man credited with helping to bring about the peace process that drew a line under the Troubles in 1998.

The BBC said it acted in good faith and that the claim was presented as an allegation and not as a fact and that it had been corroborated by five other sources, including the security services. Lawyers for the broadcaster argued that because Adams was widely considered to have been an IRA commander during the Troubles he had little reputation to lose and that any damages would be a “cruel joke”.

Speaking outside the court, Adams said: “From my perspective, taking this case was about putting manners on the British Broadcasting Corporation. I know many, many journalists. I like to think that I get on well with the most of them, and I wish you well, and I would uphold your right to do your job.

“But the British Broadcasting Corporation upholds the ethos of the British state in Ireland, and in my view it’s out of sync in many, many fronts with the Good Friday agreement. It hasn’t caught on to where we are on this island as part of the process, the continuing process, of building peace and justice, and harmony, and, hopefully, in the time ahead, unity.”

Paul Tweed, a solicitor for Adams, said his client was very pleased at the verdict, which he said showed the BBC should not have included the disputed claim in its broadcast. “Not only had the false allegation regarding our client been the focus of the Spotlight documentary, but it had been utilised to sensationalise and publicise their programme,” said Tweed.

“Furthermore, the fact that the false allegation has been left online for almost nine years has, in my opinion, done much to undermine the high standards of accuracy that is expected of the BBC. This case could and should have been resolved some considerable time ago.”

Adams, 76, took centre stage in the witness box for much of the trial, which covered his childhood and political awakening, the evolution of the Troubles, the IRA’s deadly campaign and the peace process. He repeatedly denied ever being a member of the IRA. “It wasn’t a path that I took,” he said.

Other witnesses included Jennifer O’Leary, the BBC reporter who made the documentary, and media experts who gave contrasting views on the programme and accompanying article. Litigation lawyers estimate the total cost of the trial could exceed several million euros.

Before sending out the jury to begin deliberations on Thursday, Justice Alexander Owens said there was no need to decide on the veracity of the allegation that Adams sanctioned Donaldson’s murder, or to make a judgment on the republican leader’s role in Irish history.

The judge instructed the jury of five men and seven women to consider whether the words in the documentary and article meant Adams had sanctioned the murder, or were presented only as an allegation, as the BBC had argued. The jury determined that the meaning of the words was that Adams had sanctioned the murder. It also found that the BBC had not acted in good faith and not proved its “fair publication” defence.

The judge had told the jury that if it found in favour of Adams and awarded damages, it should assess the damages on the basis of Adams’s current and recent reputation.

Adams was a teachta dála – a member of the Dáil – for Louth when the programme aired. He stepped down as president of Sinn Féin in 2018 after leading the party for 35 years.

Adam Smyth, the director of BBC Northern Ireland, expressed disappointment at the outcome. Speaking outside court, he said: “We are disappointed by this verdict. We believe we supplied extensive evidence to the court of the careful editorial processes and journalistic diligence applied to this programme, and to the accompanying online article. Moreover, it was accepted by the court and conceded by Gerry Adams’s legal team that the Spotlight broadcast and publication were of the highest public interest.

“We didn’t want to come to court but it was important that we defend our journalism and we stand by that decision. Our past is difficult terrain for any jury and we thank them for their diligence and careful consideration of the issues in this case.”