NORTHERN Ireland's power-sharing Executive has been plunged into fresh crisis after the Ulster Unionists announced their intent to walk away from the administration over claims the Provisional IRA still exists.

UUP leader Mike Nesbitt said trust in Sinn Fein had been shattered by the revelations and his party had been left with no option other than to resign from the five-party coalition and form an opposition in the Stormont Assembly.

"Without trust there is nothing," Mr Nesbitt said.

While the dramatic move by one of the three minor coalition partners will not automatically trigger the collapse of the administration, it does throw its future into serious doubt, as pressure will now mount on the region's largest party, the Democratic Unionists, to follow suit.

If the DUP left the Executive it would fold immediately.

Sinn Fein accused the UUP of cynical politicking and claimed the party was trying to contrive a crisis to gain an electoral edge over the DUP ahead of next year's Assembly poll.

The UUP decision comes after the Police Service of Northern Ireland (PSNI) said structures of the PIRA are still operating, and some of its members were involved in the murder of Belfast father-of-nine Kevin McGuigan two weeks ago.

It is almost 20 years since the Provisional IRA's last ceasefire and a decade on from the supposed decommissioning of its weapons.

The DUP is set to meet Northern Ireland Secretary Theresa Villiers to discuss the situation and is seeking an "urgent meeting" with Prime Minister David Cameron.