Rebel Ulster Unionist MP Mr Jeffrey Donaldson tonight quit the Ulster Unionist Party.
Party leader, Mr David Trimble was plunged into the biggest crisis of his leadership after his chief critic announced he was leaving the party along with two other Assembly members.
The party issued a brief statement this evening expressing regret over Mr Donldson's decision.
"Jeffrey's decision is a matter of regret but it has not come as a surprise.
Jeffrey was a leading member of the Party, he was a moderniser in the talks team right up to the last minute. Thereafter he lost his way.
We wanted him to remain in the Party, supporting the policies and democratic decisions of the Party. He clearly felt unable to do so.
Fermanagh and South Tyrone MLA Ms Arlene Foster and Lagan Valley Assembly member Ms Norah Beare are also going.
Mr Donaldson said: "We have made the decision that we are resigning from the Ulster Unionist Party and will be considering our options."
One could involve him joining the Rev Ian Paisley's Democratic Unionists.
He confirmed: "I have been offered a position on the DUP's negotiating team to participate in the negotiations taking place in January and will be taking a decision after the holidays.
"This is a sad day for me. I have been a member of the Ulster Unionist Party ever since I was a teenager.
"However I have come to the conclusion that it is not the party I joined and it has abandoned the principles I believe in."
Mr Donaldson's resignation came after over five years of in-fighting which has left the Ulster Unionist Party hopelessly split over support for the Belfast Agreement and power sharing with Sinn Féin.
Mr Donaldson made several failed attempts to change party policy the UUP's ruling council.
He has been the MP for Lagan Valley since 1997.
Ms Foster, a lawyer, was honorary secretary of the party and close ally of Mr Donaldson's.
Ms Beare has worked in the MP's constituency office and was elected alongside him in Lagan Valley during last month's Assembly Elections.
The resignations mean Mr Trimble's Assembly Group has dropped from 27 seats to 24 - the same number as Sinn Féin's.