Lady Sylvia Hermon opposes the link-up between the UUP and Conservatives
The Ulster Unionist Party has been informed by Lady Sylvia Hermon she will not be seeking the nomination as a Conservative and Unionist candidate.
The North Down MP has said she will not apply to stand for election under the joint Ulster Unionist and Conservative banner.
The UUP's deadline for nominations closed at 1700 GMT.
Sir Reg Empey and UUP Chairman David Campbell said they wished to record their regret at her decision.
In a statement, they said: "We wish to record our regret at her decision, but also our sincere appreciation for her exemplary service to her constituents in North Down, our party and the wider community, since her election as Ulster Unionist MP in 2001."
The Ulster Unionists have selected nominees in all constituencies apart from North Down.
The Ulster Conservatives and Unionists - New Force (UCUNF) electoral link-up between the two parties was forged in 2008.
BBC NI political reporter Stephen Walker said that of all the 18 constituencies in Northern Ireland, North Down has given the Ulster Unionist Party its "biggest headache".
"Lady Hermon has always insisted she is against the electoral alliance with the Tories and won't stand on a joint Conservative and Unionist ticket."
He added: "Her decision will now heighten speculation that she will run as an independent."
The UUP and Conservatives have agreed to select candidates to stand in Westminster and European elections.
In October, the Conservatives selected Ian Parsley as their choice to stand in North Down.
A former Alliance Party member, Mr Parsley joined the Conservative Party in September 2009.