MLA for Strangford Michelle McIlveen said: "I congratulate Reg Empey for finally managing to secure a political marriage. We have all watched with slightly bemused embarrassment the repeated attempts of the UUP to get hitched, first to the PUP, then to the Tories and latterly to the TUV. Now at last it seems that wedding bells are in the air."

But she said the link-up raises questions.

Will the UUP support the Tory plan of running candidates in every seat in Northern Ireland, even if it means losing Fermanagh and South Tyrone and South Belfast to anti-unionist parties?

Furthermore, she said, Sir Reg "has opted to merge with a party whose leading members have made openly offensive remarks about key elements of the unionist family" – namely comments on a local Conservative Party website by Northern Ireland Tory chairman Jeffrey Peel describing the Orange Order as a "backward facing parish pump society".

"Will such thinking be a factor in picking UUP/Tory candidates?" Ms McIlveen said.

Meanwhile, another party showing an interest in the new deal was Alliance.

North Down MLA Stephen Farry questioned the rationale of David Cameron's Conservatives in signing up to a pact with the Ulster Unionists that "puts identity politics ahead of their apparent mission of building modern, progressive politics".