David Trimble, a key backer of Northern Ireland's 1998 peace accord and a Nobel Peace Prize laureate, said Saturday that he would resign as the leader of the Ulster Unionist Party after losing a decade-long battle to steer fellow Protestants toward compromise with Roman Catholics.

Trimble's decision to quit followed the Ulster Unionists' worst-ever performance in British parliamentary elections Thursday. The party, which once dominated politics in this predominantly Protestant territory, retained just one of Northern Ireland's 18 seats, and the highest-profile loser was Trimble.

The Democratic Unionist Party, led by firebrand preacher Ian Paisley, 79, ate into the Ulster Unionists' support, winning nine seats including Upper Bann, a district southwest of Belfast that Trimble had held since 1990.

In a statement issued by Ulster Unionist headquarters in Belfast, Trimble, 60, said he told senior colleagues in his party that "I do not wish to continue as leader."

Trimble's most likely successors also suffered defeats Thursday against the hard-line Democratic Unionist Party, which has mobilized growing Protestant hostility to the 1998 accord and Trimble's long-pivotal support for it.

Analysts credit Trimble with taking many gambles concerning the complex Good Friday pact, which proposed dozens of goals designed to promote peace and reconciliation following a three-decade conflict over this British territory that left 3,600 dead. Today, Northern Ireland is deeply polarized but largely peaceful.

Trimble narrowly survived several attempts to unseat him as Ulster Unionist leader, but he could not stop the flow of votes to Paisley's Democratic Unionists.

"History will show that without David Trimble's period as leader of the Ulster Unionist Party, Northern Ireland could never have become the changed place it is today," said British Prime Minister Tony Blair, who won a third term Thursday. "Without him . . . it would not have been possible to bridge the deep divide in Northern Ireland."

While other Protestant parties boycotted negotiations involving Sinn Fein, the party linked to the outlawed Irish Republican Army, Trimble kept the Ulster Unionists at the table. Trimble backed the deal on April 10, 1998, -- and immediately split his party down the middle.

The pact called for Protestants and Catholics, including Sinn Fein, to form a power-sharing administration; for convicted members of the IRA and other truce-observing paramilitary groups to be freed; and for those groups to disarm fully by mid-2000.

Protestants expected the IRA to disarm as the price for Sinn Fein's involvement in government, but the IRA refused. Trimble -- who received the Nobel prize alongside Catholic moderate leader John Hume -- rallied Protestant support with the slogan, "No guns, no government."

But after a 17-month deadlock, Trimble announced he would "jump first" and formed, in December 1999, a four-party coalition that included Sinn Fein. To the disgust of many Protestants, the administration included a reputed IRA commander, Martin McGuinness, as education minister.

Britain soon freed remaining IRA prisoners and began a painstaking reform of Northern Ireland's mostly Protestant police force -- moves opposed by Protestants -- but the IRA refused to disarm in reply. Trimble repeatedly took the administration to the brink of collapse in showdowns with the IRA, which finally scrapped an unknown amount of weaponry in October 2001.

But the delay, and the IRA's insistence that its handover of arms remain secret, proved deadly to Trimble's political base.

David Trimble, Ulster Unionist Party leader and Nobel winner.