David Trimble, Northern Ireland's first minister, has upped the stakes in Northern Ireland by threatening to resign unless the IRA begins to decommission its weapons.

Making a personal statement in the assembly on Tuesday, Trimble announced that he had "reluctantly" given a letter to the speaker, Lord Alderdice, resigning as first minister with effect from July 1.

The letter will take effect unless "the republican movement keeps its promise" to disarm, he told assembly members.

The Good Friday peace agreement set June as the deadline for the disarmament by terrorist groups - so far the only weapons handed in have come from the Loyalist Volunteer Force.

Last May, an IRA statement that followed negotiations at Hillsborough Castle said the organisation would begin to put its weapons "completely and verifiably" beyond use.

Since then there have been two inspections of arms dumps by international arms inspectors, Cyril Ramaphosa and Martti Ahtisaari. Following the inspections, the IRA broke off contact with General John de Chastelain's decommissioning body, before resuming discussions in March.

"Because there is so little time left, because some in the republican movement think they can avoid their obligations and because there have been comments purporting to come from government casting doubt on that date, I have decided to reinforce the agreement made at Hillsborough," said Trimble.

With republicans rejecting any unionist attempt to force the IRA into decommissioning and the Democratic Unionists mounting a vigorous campaign against the peace agreement, the Ulster Unionists have been under increasing pressure on the weapons issue.

The renewed trouble for the peace process comes as the DUP prepare a motion of no confidence in education minister Martin McGuinness, following his admission that he had been a senior IRA commander at the time of the Bloody Sunday killings. To be debated on Tuesday afternoon in the assembly, the no confidence motion is unlikely to gain sufficient cross-community support to be carried.