Splinter group from Provisionals had been planning armed action

Bombers' leader issued earlier threats against the military

Gardai intelligence sources believe the bomb was the work of a nationalist splinter group known as Republican Sinn Fein.

The group, which split from the Provisionals in 1986, had been planning armed actions, the sources said.

Their suspicions are based on known attempts by the small organisation to form a military wing. The group last year boasted that it had recruited some dissident IRA hardliners who were opposed to the then provisional ceasefire that was declared in September 1994.

At its annual conference held in Dublin last November, Republican Sinn Fein (RSF) - headed by a former Provisional IRA leader, Ruairi O'Bradaigh - endorsed terrorist attacks on British military targets and made no secret of the fact that there was a military wing within the group.

That warning came just two days after 2,000lb of explosives had been discovered south of the border with the Irish Republic in Co Monaghan. Security sources linked the discovery to members of the RSF.

In Garda raids carried out throughout the Irish Republic in October 1994, RSF supporters were singled out and pistols, ammunition, hand grenades and radio equipment were seized.

The RSF conference endorsed "the right of the Irish people to use whatever degree of controlled and disciplined force (is) necessary in resisting English aggression".

In a calculated side-swipe at talk of IRA decommissioning, it also condemned "any surrender of arms obtained for the above".

A Dublin RSF spokesman told the Independent that it had recruited experienced Provisional IRA members from units in Belfast, Dublin and Derry.

The group had repeatedly denounced the ceasefire declared in 1994 by the IRA as a "surrender". The Irish authorities took the RSF's attitude seriously and last year mounted a wide-ranging campaign against its members and its suspected military wing to head off a perceived threat to the Ulster peace process.

Mr O'Bradaigh maintained that the IRA's ceasefire had provided a "window of opportunity" for the more militant splinter group to start a new military campaign against the British presence in Northern Ireland.

Yesterday the IRA issued a statement denying it had carried out the Enniskillen attack.

Sinn Fein and Provisional IRA leaders have been keenly aware of the danger of carrying out bomb attacks in Northern Ireland that could jeopardise the delicately balanced loyalist paramilitary ceasefire and plunge the province into full-scale civil war.

The nationalist INLA, riven by a violent internal feud and weakened by the seizure outside Dublin in April last year of much of its arsenal, is not believed capable at present of mounting a major bombing campaign.

Gardai also noted that the Enniskillen bomb followed just hours after an IRA appeal for calm, suggesting it would use force only if nationalist areas came under further attack.

Londonderry sources say known IRA figures early on Friday ordered young would-be arsonists out of a store in the city centre which has experienced an economic and tourism renaissance in recent years since extensive restoration.