Ulster Unionist leader Doug Beattie has said it was "clear" that a meeting he had with Sir Jeffrey Donaldson last year was to discuss the DUP MP potentially switching parties.
Mr Beattie said he has texts of exchanges with Sir Jeffrey and signalled he may publish the messages if his integrity is challenged.
Sir Jeffrey, who later became DUP leader, has denied reports that he considered rejoining the UUP after he lost out in a leadership battle to Edwin Poots.
Read more: Donaldson says he makes no apologies for engaging with other unionists
It was claimed that a "top secret" meeting between Sir Jeffrey and Mr Beattie was held at an Army barracks with talks lasting more than 90 minutes.
Sir Jeffrey last week said the "portrayal" that he had discussions about rejoining the party he left in 2003 was "nonsense".
The Lagan Valley MP said he was approached by the UUP but "respectfully declined" and any discussions he had focused on achieving "closer unionist cooperation".
But Mr Beattie said he initiated the meeting last June and it was organised to discuss Sir Jeffrey joining his party.
Speaking in Tandragee, Co Armagh, Mr Beattie said: "The invite was quite clear, the invite was that I would welcome him to have discussions with me about rejoining the Ulster Unionist Party. That’s what it was about.
"That's what it was sent for and that’s why we organised a meeting to talk about that.
"I relayed that to my party chairman (Danny Kennedy) and we had that meeting. And it was a long meeting and we talked about it and we talked about all of the issues that we faced, but that’s what it was about.
"And it didn't come to fruition because circumstances overtook us."
Soon after the meeting, Sir Jeffrey became DUP leader following an internal party revolt against Mr Poots.
Mr Beattie said the meeting did not result in a definitive outcome on whether Sir Jeffrey intended to rejoin the UUP.
He also said the unexpected end of Mr Poots' tenure as DUP leader meant the engagement with Sir Jeffrey went no further.
"Had he not become the leader of the DUP, I’ve got absolutely no idea where it would have went," he added.
Asked whether he would publish text exchanges with Sir Jeffrey if his version of events was disputed, Mr Beattie said his integrity was important to him.
While he said he did not plan to publish the texts, he added: "But if people are going to challenge me, then I will do everything that I need to do to make sure that they understand that I’m an honest politician and I’m telling the truth."
Earlier, DUP Economy Minister Gordon Lyons said talk of his party leader potentially rejoining the UUP last year was "tittle tattle".
Mr Lyons said he and other members of the DUP he had spoken to accept Sir Jeffrey's insistence that he never had any intention of joining the Ulster Unionists.
"I don't think that the public are interested in what happened a year ago or who said what, even now," he told reporters in Belfast.
"They're looking to the future. If you listen to what people are actually concerned about right now, it's the future, it's about cost of living, it's about what comes next for them. That’s where our focus is right now as well."
Asked if he had contacted Sir Jeffrey to seek assurances on the issue, Mr Lyons added: "No, I've had no need to raise those concerns. Jeffrey Donaldson set out his position and I can tell you speaking to other people in my party, they accept that as well and want us to actually focus on what comes next rather than tittle tattle from the last year."
Read more: Donaldson says he makes no apologies for engaging with other unionists
Read more: DUP MP criticises claims Sir Jeffrey Donaldson considered UUP switch
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