Veteran Ulster Unionist politician Chris McGimpsey is suing his own party for breach of contract.
The dispute centres on May's council election when the UUP ran both Mr McGimpsey and former Tory Ben Manton in the Lisnasharragh ward in south east Belfast.
Mr McGimpsey was opposed to standing a second candidate as he said there was only one UUP seat in the ward.
He secured 508 votes to his running mate's 415. He lost his seat on the last count and Mr Manton wasn't elected.
It is understood that the legal action centres on an alleged verbal agreement between the UUP veteran and his party over the conduct of the campaign.
Earlier this week, Belfast High Court heard that Mr McGimpsey is to pay £63,000 for falsely referring to Derry-born Republic of Ireland player James McClean as a "super-Provo". His legal action against the UUP pre-dates that settlement.
Mr McGimpsey has been a UUP member for 52 years and retains membership of the Knock and Ballyhackamore branch.
He had been a councillor for almost two decades representing Court and Castlereagh Central before being elected in Lisnasharragh in 2014.
Both Mr McGimpsey and the UUP yesterday declined to comment. The veteran politician is also suing Mr Manton for libel. Mr McGimpsey alleges that his running mate defamed him on social media.
Mr Manton had tweeted in response to an interview Mr McGimpsey had given prior to the count in which the councillor said he didn't know Mr Manton.
"He was in the Conservative Party and he joined us I think about a year ago so I don't really know the guy. I've spoken to him once I think," Mr McGimpsey said.
Mr Manton then tweeted that he wanted to "clarify" that he had been a member of the UUP "since January 2017" and he went on to make other remarks Mr McGimpsey alleges were defamatory.
Mr McGimpsey is one of his party's most left-wing and liberal members. He has long been pro-choice on abortion and is a staunch animal rights supporter and critic of Belfast Zoo.
Belfast High Court heard on Wednesday that the veteran UUP man will pay the compensation to James McClean in two instalments. Mr McGimpsey had already issued an unreserved apology for the remarks which he said were made "in the heat of the moment" in an interview with BBC Radio Ulster's Nolan Show last year.
He also paid substantial damages to Sinn Fein councillor Ciaran Beattie for "false and unfounded" Facebook comments about him last year.
Mr McGimpsey and his older brother Michael rose to prominence in the mid-1980s when they challenged the Anglo-Irish Agreement in the Dublin courts.
Speaking previously about the legal action, he said: "The lawsuit was very much my baby. It cost a fortune.
"I linked up with (Lord) Ken Maginnis, who launched a campaign to raise money for the case and there was enough money raised to pay the legal bills."
Although they lost, he said it was worthwhile.
"The idea was either to get the Anglo-Irish Agreement declared unconstitutional, which the Irish state was never going to do, or undermine the credibility of Articles 2 and 3 of the Irish Constitution, and that's what we achieved," he said.
"In its editorial the morning after the court decision, the Irish Times wrote: 'McGimpsey lost the battle but won the war'."