Range of extremist groups in the frame

Launching a car bomb attack on London during this momentous week in British politics would have been attractive to a wide range of extremist groups.

The Haymarket incident seems most likely to be connected with al Qaida - but could be the work of dissident Irish Republicans or even animal extremists.

Police will have a good idea which group might have been responsible from the style of device, its construction and the type of explosive used - but none of these details has yet been made public.

If reports that it contained home-made explosive are correct, it would most likely be a hydrogen peroxide mixture as used in the July 7 atrocities, although a small "fertiliser bomb" made of ammonium nitrate cannot be ruled out.

Counter-terrorism expert Professor Paul Wilkinson of St Andrews University said: "We have to keep an open mind about who may have been responsible at the moment because we know so little about the nature of the device."

He said it was of "great concern" that police were reportedly alerted to the car bomb by a member of the public.

"That indicates that the police did not have any indication that there was a conspiracy to set off an explosion. That is of great concern.

"The police have recently been very successful in getting intelligence and pre-empting attacks.

"In this case the device was already in the West End, presumably ready to be detonated."

A car bomb would be a new tactic for al Qaida groups in the West, who have recently concentrated on suicide-bombing tactics in their attempts to kill as many innocent people as possible.

If they have adopted the car bomb, it will be a worrying development for the security services because by its very nature the terrorist is not killed, as in a suicide bombing, and can go on to plot more explosions, increasing the likelihood of a sustained campaign.

If they succeed in their murderous aims, a small cell of terrorists using car bombs could achieve far more destruction than if they had died in their first wave of attacks.

He said: "Even if it was a smaller device than those set off in the Middle East, if it was detonated in the crowded West End, perhaps when people were commuting at rush hour, it would certainly have killed and injured people, as well as damaging property."

Bomb disposal experts will check the device for construction methods used by dissident republicans in Northern Ireland as well as other armed groups across the world.

Factors such as the presence of a timing device, the use of a home-made explosive or the much more potent Semtex - 5lb of which could destroy a street - will be vital indicators.

Features including the soldering of wiring or the casing for the device, often linked to a battery to provide a spark, will be compared with other incidents across the UK as detectives try to find the culprits.

London is no stranger to IRA onslaught. The image of royal horses blown apart during the July 1982 Hyde Park bombing is hard to forget.

Dissident republicans are bitterly opposed to the May 8 power-sharing peace settlement in Northern Ireland between Sinn Fein and the Rev Ian Paisley's Democratic Unionists.

They have launched attacks before, most notoriously in August 1998 when 29 shoppers and tourists died on the streets of Omagh, Co Tyrone, in a Real IRA blast.

Although the Army is ending its support role for the police in Northern Ireland - General Officer Commanding Nick Parker said on Monday that the dissident threat did not merit a military presence - there have been a series of firebombings at retail warehouses linked to disgruntled activists.

The fringe element of republicanism is unhappy with the continued presence of the British Government in Ireland, wants a united Ireland and accuses Sinn Fein of selling out on its core values.

But it has been frustrated in a string of failed attacks in Banbridge, Londonderry and Armagh and is riddled with informers linked to the police.

Prof Wilkinson said of the possibility of dissident Republican involvement: "We know that they still exist and that they are diametrically opposed to the peace process and power-sharing institutions."