THE Northern Ireland peace process was engulfed in a fresh crisis last night when David Trimble stepped down from the executive, claiming he would only return if the IRA disarms.

Mr Trimble resigned as First Minister at midnight on Saturday, arguing the IRA had failed to honour its commitment to put its weapons beyond use.

The Ulster Unionist leader, who was in France for a Battle of the Somme commemoration, said he could no longer believe the IRA because it had missed two arms deadlines in May last year and also last night.

He said: "I am prepared to resume that office, but only if we get this issue settled and we see weapons being put permanently beyond use in accordance with the decommissioning legislation."

Mr Trimble's resignation has plunged Northern Ireland's political institutions into turmoil at a sensitive time in the peace process, with tension mounting on the streets in the run-up to next Sunday's Orange march at Drumcree.

The British and Irish Governments and the parties have only six weeks to resolve problems over decommissioning, policing, demilitarisation and the full operation of the institutions.

At the end of those six weeks, Mr Trimble must seek re-election in the assembly to the post of First Minister, a move which will require the support of 50 per cent of unionist MLAs, plus one and 50 per cent of nationalists, plus one.

The province's nationalist Deputy First Minister, Seamus Mallon, whose office is interlinked, will also have to be re-elected.

Failure to elect First and Deputy First Ministers would result in fresh assembly elections.

However, Northern Ireland Secretary Dr John Reid could suspend the institutions before the vote in a bid to buy more time to find a deal.

Dr Reid and Irish Foreign Minister Brian Cowen are expected to begin talks with the parties later this week.

Following his departure, Mr Trimble's functions have been transferred to Enterprise Minister Sir Reg Empey to enable the Executive to meet over the six weeks. Mr Mallon will continue to perform his duties.

Mr Mallon responded to pressure on his party to expel Sinn Fein from the executive by insisting this was not a matter for the SDLP alone.

He said: "You are asking me a hypothetical situation, which ignores the section of the agreement in the legislation which gives the Secretary of State the power to require the Speaker to put a motion in the assembly. That has not happened."

Sinn Fein national chairman Mitchel McLaughlin warned if that happened, it would involve "tearing up the Good Friday Agreement".