In response to “romantic overtures” from the DUP, he launched a stinging attack on Peter Robinson – during his speech to the Ulster Unionist annual general meeting at the King’s Hall complex on Saturday.

The UUP leader expressed incredulity at the new DUP leader suggesting, at the weekend, that the idea of one unionist party was now on the table – post-Paisley.

Mr Robinson and colleagues had spent years splitting unionism for their own ends and now wanted to unite when it suited them, said Sir Reg.

The DUP knew its vote had peaked and would decline under Traditional Unionist Voice pressure, he indicated. And Mr Robinson was scouting around for ways to protect his power base.

“Let me put it bluntly,” said Sir Reg. “The Ulster Unionist Party didn’t endure the orchestrated abuse against us since 1997; or the tidal wave of attacks upon our integrity; or the vilification of our leading figures simply to strike up a marriage of convenience with a DUP which is worried about the impact of Jim Allister.”

Pointing the finger at the DUP for engaging in a nasty brand of politics, he said: “He (Mr Robinson), personally ran the propaganda machine that spat out most of the venom at this party.”

And to loud applause from more than 400 delegates, the UUP leader said: “Too many of us know too much about the DUP to be easily swayed by romantic overtures at this stage.”

Sir Reg did, however, say he was up for discussions about cooperation between the two sides, on maximising the unionist vote.

But again warned that Mr Robinson’s continued use of the media to raise the matter smacked of a PR “stunt” rather than a serious attempt to do business.

He claimed the DUP could talk of mergers because they had adopted UUP policies already and done what they said they would never do.

Sir Reg said: “I am often asked if it annoys me that the DUP, in March 2007 – having hounded us every step of the way – immediately repudiated almost every policy and mantra they ever espoused and somersaulted their way into an Executive structure that contains five Sinn Fein ministers, rather than the two we had in 1998?

“Ladies and gentlemen, annoyed doesn’t begin to sum it up!”

He continued: “I am sick of the DUP now claiming the credit for everything good that has happened in Northern Ireland since 1998. Particularly when they did everything they could to undermine the efforts we were making.”

Ulster Unionists he admitted “may have made mistakes”. But they had taken the risks when the DUP said: the Irish government would never accept an internal settlement; Sinn Fein would never buy into partitiom, and the IRA would never disarm.

“At least no one can accuse us of hypocrisy and sheer naked opportunism,” he declared.

“I believe that we have a legacy of which we can be proud.