Former DUP health minister Jim Wells has produced an email that he claims proves he had a commitment from ex-leader Peter Robinson that he could return to his post.
Mr Wells, who is involved in a bitter fallout with senior party colleagues, has claimed the DUP reneged on a promise to reinstate him as health minister after his controversial resignation three years ago.
He said the then leader Mr Robinson said he could return but alleges the current leadership failed to honour that commitment.
Last week Mr Robinson denied he had made a specific promise that Mr Wells could return to the health portfolio, insisting in a BBC interview that his words were "much more careful".
In the latest development in the saga, Mr Wells has produced an email he says was sent by Mr Robinson to a party official in 2015.
The email, published in the News Letter, quotes the then DUP leader as saying he "would be happy to see him back in government" and that "no impediment was placed in his being able to return to office".
Mr Wells insisted the email indicates a promise was given.
"I regret that I have been forced to release private correspondence between myself, the former party leader and the party officers but this had to be done to confirm that I have told the truth about how I have been treated," he said.
Mr Wells, who has been a DUP member for 43 years, an MLA for 24 years and was a councillor for 11 years, is among the longest serving public representatives at Stormont.
He was appointed health minister in 2014 but stepped down the following April after a furore around comments he made at a hustings event.
Mr Wells said he had been falsely accused of linking same-sex marriage to child abuse but had fallen on his sword to protect the party ahead of the 2015 general election.
He said he should have subsequently been offered to return to ministerial office.
A DUP spokesman said: "The party is aware of the comments. This is an internal matter, therefore the party will not be giving a running commentary."