Northern Ireland's largest Protestant party announced Thursday it was resigning from the province's unity government, a move designed to force Britain to indefinitely suspend the troubled experiment in Catholic-Protestant cooperation.

Ulster Unionist leader David Trimble said his party's three remaining Cabinet ministers had handed in resignation letters to take effect at midnight.

Trimble, who precipitated the crisis by resigning in July as the government's leader, said his party had spent 18 months operating a coalition that included militant Catholics from Sinn Fein — but the party's Irish Republican Army allies had failed to meet their end of the bargain by disarming.

"We have sustained an inclusive executive for 18 months. For 18 months we have demonstrated every day our willingness to make progress in terms of this institution and politics in Northern Ireland," said Trimble, who shared the 1998 Nobel Peace Prize with Catholic moderate leader John Hume.

"And for those 18 months the republican movement have done nothing, nothing at all to reciprocate the sacrifice and risks we have made," Trimble said, using the umbrella term for Sinn Fein and the IRA.

Under terms of the law governing how Northern Ireland's power-sharing government operates, it cannot survive without the participation of either the Ulster Unionists or the largest Catholic-supported party, Hume's Social Democratic and Labor Party.

Trimble said Britain's secretary of state for Northern Ireland, John Reid, had a week to decide whether the administration should have its powers stripped again — effectively putting the four-party coalition into cold storage — or dissolving it completely to prepare for new elections.

Reid, who was scheduled to discuss the crisis Friday with the Irish government, has previously said he didn't want to risk a new election in Northern Ireland. Voters in June gave unprecedented levels of support to the two most hard-line parties in the coalition, Sinn Fein on the Catholic side and Ian Paisley's Democratic Unionists on the Protestant side.

Britain has already twice taken power from local hands for brief periods to forestall deadlines to re-elect Trimble as the Protestant leader. Political analysts say Trimble doesn't have enough support among Protestant lawmakers to win a leadership vote, chiefly because of the IRA's refusal to scrap its weapons as the 1998 peace accord envisaged.