There are three main groups, each formed from splits in the movement over the last three decades.

The main movement has split a number of times

The most dangerous appears to be the most recent to split away.

The "Real IRA", which is blamed by many for this attack, is thought to operate from the Republic of Ireland.

The group is believed to have been responsible for a series of recent attacks, including a car bomb which devastated the market town of Banbridge, Co Armagh, earlier this month.

In May, following a mortar attack on a police station in Co Fermanagh, the "Real IRA" declared that it appointed an "army executive" and that a "war machine is once again being directed at the British Cabinet".

It is believed the "Real IRA" has access to arms dumps

The group is led by the man who resigned as the IRA's quartermaster-general last October in protest at the peace process.

He lives in Dundalk, Co Louth, in the Irish Republic, and is thought to have taken with him a number of the IRA's bombmakers - as well as details of arms dumps.

This has increased the pressure on Sinn Fein to ensure that weapons are handed over before they get into the wrong hands, although the security forces insist that they do not believe the recent bombings have not been assisted by the IRA proper.

Membership growing

Reports put the membership of the "Real IRA" - also based largely in the Republic - at anything up to 100, and growing.

This car bomb in Banbridge was thought to be a "Real IRA" attack

Just three days before the Omagh attack the Republic's police force, the Garda said most of its counter-terrorist resources were now devoted to fighting the "Real IRA".

Bomb-making equipment had been seized and an apparent 'test' bomb blast was discovered when animals were startled and a crater discovered in farmland.

It is believed this was caused by a bomb of up to 1,000lbs set off by a paramilitary group hoping the sound would have been masked by a local fireworks display.

Protestant Billy Wright was killed by the INLA

Police south of the border also fear that the "Real IRA" is forming links with the other two dissident groups - the Continuity IRA, and the Irish National Liberation Army (INLA).

The INLA was formed in 1975, mainly from disaffected members of the IRA unhappy at a previous ceasefire.

It was responsible most recently for the murder of leading loyalist Billy Wright inside the Maze Prison on 27 December.

Work of the Continuity IRA: Hotel bomb in Enniskillen in 1996

The Continuity IRA is believed to be linked with Republican Sinn Fein, which split off from the main party in 1986, though that party denies having a military wing.

The security forces have linked it to attacks this year such as the Moira and Portadown bombs in February.

Action against the dissidents is now likely to be stepped up, concentrating on border areas, with more arms searches and surveillance of known sympathisers.

It has also been revealed by the security forces that a leading member of Sinn Fein from West Belfast had talks with the leader of the "Real IRA" and urged him to end the campaign.