The Ulster Unionist Party's only MP has quit the party and announced she will stand as an independent at the general election.
Lady Sylvia Hermon objected to the UUP's electoral pact with the Conservatives, and had previously said she would not stand under a Tory banner.
She was first elected as MP for North Down in 2001 and re-elected in 2005 with more than 50 per cent of the vote.
The 57-year-old former law lecturer at Queen's University Belfast was a strong supporter of the Good Friday Agreement and devolution of powers to the cross-community devolved administration.
"As we approach the end of this parliament with the Easter recess beginning next Tuesday and dissolution expected shortly thereafter, the mandate given to me at the last general election to serve the people of North Down as an Ulster Unionist is virtually at an end," Lady Sylvia said today.
"The distance, however, between me and my party has become so great over recent months that I have decided to resign from the Ulster Unionist Party.
"I have taken a considerable amount of time to think and to reflect before coming to this decision, but I believe it to be the right one for the party and for me.
"The tremendous support I have received from constituents during the recent stressful and difficult time in my political life has encouraged me greatly, and convinced me that I ought to run again as a candidate."
Lady Sylvia's husband Sir John Hermon was chief constable of the Royal Ulster Constabulary from 1980 to 1989.
He died in 2008 after a long struggle with Alzheimer's disease.
Lady Sylvia remains a popular figure in her constituency, and the Belfast News Letter reports there is "growing speculation that the DUP will not stand in the Westminster election in North Down".
In 2001 she was supported by the Alliance party, who chose not to run a candidate against her.
The UUP/Conservatives have chosen the former Alliance candidate in the Euro elections, Ian Parsley, as their candidate for North Down.
UUP leader Sir Reg Empey said he "deeply regretted" Lady Sylvia's decision and thanked her for her work since 2001.
Dods Parliamentary Communications Ltd