Lady Sylvia Hermon said she was not a Conservative supporter
The sole MP representing the Ulster Unionist Party, Lady Sylvia Hermon, has resigned from the party and will contest the election as an independent.
Lady Hermon said she was leaving the party, having refused to support its pact with the Conservatives.
In a statement, she said the distance between herself and the party had become "so great" that she had decided to resign with "profound sadness".
She has served North Down since 2001 and said she would "stand her ground".
She added: "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 Hermon had previously stressed that she would not stand on the joint ticket.
"I'm not a Tory - I'm not going to stand under the Tory-Ulster Unionist banner - it's as simple as that," she said.
Ulster Unionist leader Sir Reg Empey said he deeply regretted Lady Hermon's decision to leave the party.
"I wish to thank her for her contribution to Ulster Unionism since her election in 2001," he said.
"I believe that the values we share far outweigh the differences in approach to some issues.
"The party's major policy decisions have been reached after widespread consultation and have been unanimous."
The Ulster Conservatives and Unionists - New Force (UCUNF) electoral link-up between the two parties was forged in 2008.
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.