Sylvia Hermon said she was "disappointed" at the Tory-UUP link up
The Ulster Unionist's only MP, Sylvia Hermon, has indicated that she will not stand under the party's new banner at the next general election.
The UUP has linked up with the Tories to contest elections as Ulster Conservatives and Unionists, but Lady Hermon said she was "disappointed".
"At the present time, I can't see myself standing under a Conservative banner," she said.
UUP leader Sir Reg Empey said the party were dismayed at her stance.
"The leadership of the Ulster Unionist Party is disappointed that Lady Hermon, in the middle of an election campaign, has chosen to give a series of interviews in which she has challenged party policy and the collective decisions taken by her colleagues," he said.
"Her own position is not an issue in the present campaign.
"She was assured last year that she would not be required to take the Conservative Party whip in the lifetime of the present parliament.
"The Ulster Unionist Party and Conservative Party will continue with our joint campaign to bring real change to politics in Northern Ireland."
Lady Hermon said she was elected as an Ulster Unionist and would serve under that mandate.
Sir Reg Empey said he the party leadership were disappointed in Lady Hermon's stance
"If my party chooses to move to call themselves by a different name, I'm terribly sorry and terribly disappointed by that, but I remain an Ulster Unionist," she said.
"That was certainly my mandate and I've loved serving the people of North Down.
"They have stood by me through the most difficult of times and if they choose and wish me to serve them I would do my very best to do that."
BBC NI political correspondent Gareth Gordon said: "We know she is not a natural Tory - if anything she leans towards Labour.
"It is not a surprise, but the fact is she has now said it three weeks before an election which is the first test of this electoral tie up.
"The party leadership, I think, she already knows will be furious that she has spoken."
Earlier on Wednesday, Lady Hermon revealed her "upset and embarrassment" at being over paid almost £3,000 in parliamentary expenses.
She said she only discovered the mistake after contacting the House of Commons fees office on Monday.