Ulster Unionist leader Mr David Trimble today challenged former colleague Mr Jeffrey Donaldson to resign his Westminster seat and face the electorate following his defection to a rival party.
Mr Trimble went on the offensive against Mr Donaldson following his decision to switch parties to the Rev Ian Paisley's Democratic Unionists.
The UUP leader said it was up to Mr Donaldson to decide whether he would do the honourable thing by resigning his House of Commons seat and facing the voters as a Democratic Unionist.
Mr Trimble said: "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.
"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," Mr Trimble said.
"A lot of people in Lagan Valley [Mr Donaldson's constituency] feel there should be a by-election. I think it is a matter for Mr Donaldson's honour and integrity. It is up to him to decide. It lies in his hands."
Mr Donaldson confirmed his defection to the DUP yesterday along with two other former Ulster Unionist Assembly members, Ms Arlene Foster and Ms Norah Beare.
Mr Trimble said he had switched parties because he felt the Ulster Unionist leadership no longer shared core unionist values and because it had engaged in a vindictive campaign against him and other members of the party who were opposed to the Belfast Agreement.
PA