Sir Reg and Mr Cameron have both rejected a pact between the Conservatives and Unionists and the DUP, arguing that the long-term future of the union is better served by persuading those who don't vote for unionist parties – especially Catholics – to vote for a pro-union party.

But Mr Robinson says: "Is Reg Empey content that we should send Michelle Gildernew back to Fermanagh and South Tyrone as Member of Parliament because he refuses to recognise that a deal would be worth doing with the Democratic Unionist Party?

"Is he content that a nationalist could win South Belfast when he knows that a single unionist candidate can win that seat?

"I think that it's a betrayal of unionism for any unionist leader to say 'I'd rather have my nationalist opponents, who oppose Northern Ireland being part of the United Kingdom, winning a seat, rather than come to terms which would be mutually beneficial with another unionist party'."

Asked about the chances of Jim Allister's TUV being brought into any pact, he says that "nobody believes" other TUV candidates will do as well as Mr Allister but adds: "We're happy to talk to anybody if they have proposals but unless the UUP is willing to be part of that dialogue there isn't much benefit."