David Trimble has expressed his anger following Jeffrey Donaldson's decision to defect to the Democratic Unionist Party.

The Ulster Unionist leader said his party had been "inundated" with phone calls from members of the public angered by the decision.

His comments followed Monday's announcement that Donaldson, along with Stormont assembly members Arlene Foster and Norah Beare, was joining the party led by Rev Ian Paisley.

Trimble said Donaldson should resign his Westminster seat and allow the electorate to judge his decision to switch parties.

"We were inundated with phone calls yesterday by people very angry that the votes they had cast [in November's assembly elections] for the Ulster Unionist Party had now wandered off into another party," he said.

"They feel they have been let down. They feel deceived. I think this is most unfortunate that people were not more honest with the electorate."

Trimble also said that a by-election decision was "a matter for Mr Donaldson's honour and integrity".

"He is an Ulster Unionist. Without the Ulster Unionist Party he would be nobody," Trimble added.

But Donaldson later hit back at the comments, and called on Trimble to resign as UUP leader.

"Mr Trimble seems to ignore the fact that it was he and a small number of his supporters who orchestrated the move to have me expelled from the party," Donaldson said.

"Is he suggesting that if I had been expelled there should have been a by-election?"

And Donaldson added that he had received the support of his constituents since announcing the decision.

"I am aware of the deep unhappiness with Mr Trimble's leadership that still exists within the Ulster Unionist Party," he said.

"There are senior members of the party and up to as many as 10 of their assembly team that are deeply unhappy with his leadership.

"I think it is up to Mr Trimble to do the honourable thing and step down as the leader of the Ulster Unionist Party."