David Trimble has announced that he is to resign as leader of the Ulster Unionist Party.
David Trimble is stepping down from the UUP leadership
One day after losing his Westminster seat in Upper Bann, Mr Trimble said he did not wish to continue in the post.
The party lost four of its MPs in the general election. It now has one MP, compared to nine Democratic Unionists.
"I am pleased to have had the privilege of leading what I regard as the best and most democratic political grouping in Ulster," he said in a statement.
"The party chairman will be arranging for a meeting of the party executive to make the arrangements for a special meeting of the Ulster Unionist Council to elect a new leader at which point I shall formally resign."
Mr Trimble, the former first minister in the suspended Stormont Executive, is to hold a news conference on Monday.
When he seized the leadership of the party unexpectedly in 1995, his party held 10 seats at Westminster, compared to the DUP's two.
BBC political correspondent Martina Purdy said the former Upper Bann MP's 10 years at the helm of the UUP had been the most controversial in the party's history.
"With the signing of the Good Friday Agreement in 1998 he held 70% approval, but his party began to split and with the row over guns and government his base ebbed away," she said.
"At times his survival as leader was extremely marginal, with support at just over 50% from his ruling council."
It is thought a contest for the party's leadership could be held in the aautumn.
Prime Minister Tony Blair said it would have been impossible to bridge the political divide in Northern Ireland without Mr Trimble's period as leader of the Ulster Unionist Party.
"Throughout he showed real leadership - vision, courage and a sheer dogged determination despite all the difficulties involved in the search for peace," he said.
"Without him, there would have been no Belfast Agreement."
In 1998, Mr Trimble and former SDLP leader John Hume won the Nobel Prize for their contribution to the Northern Ireland peace process.
There is speculation Mr Trimble could be offered a seat in the House of Lords in the coming days.
SDLP leader Mark Durkan said Mr Trimble deserved credit for "concluding the Good Friday Agreement and reaching an historic deal with Irish nationalists".
"The pity is that while he fought the battle within unionism for the Agreement, he never went for its wholehearted implementation," he said.
Jeffrey Donaldson, who joined the DUP from the Ulster Unionists in January 2004, said Mr Trimble's resignation was inevitable after the scale of his party's election defeat.
"The demise of the Ulster Unionist Party is largely his responsibility," he said.
"His policy had become so out of touch with the unionist electorate, I think he had no alternative."
Lord Maginnis of the Ulster Unionist Party said Mr Trimble would be remembered for the "very positive contribution he made to ensuring democracy dominated and not violence".
"He moved Northern Ireland in 10 years from a backwards attitude to something that is acceptable across the world."
Alliance leader David Ford praised Mr Trimble's role in moving unionism forward and in negotiating the Good Friday Agreement.
"After the moribund years of the 80s and early 90s, David Trimble had the vision and the political courage to lead his party, and a large section of this community, to reaching the Good Friday Agreement. "
Before he announced his resignation, Mr Trimble said the basic underlying cause of the Ulster Unionists' problems had been the republican movement "not behaving properly over the years".