Whitehall was facing the prospect of a lengthy hostage stand-off last night after five Britons were kidnapped in central Baghdad in one of the most brazen abductions of Westerners since the fall of Saddam Hussein.

Those seized, a computer consultant and his four security guards, were taken in daylight by dozens of armed insurgents dressed in the fatigues of Iraqi police commandos.

The Cobra emergency committee, with representatives from MI6, the SAS and the Metropolitan Police, met at the Cabinet Office yesterday to consider options for gaining the release of the five men.

Tony Blair, on a trip to Libya, said: “We will do everything we possibly can to help.”

The committee’s priority will be to establish which group was behind the kidnapping, as their identity will determine how the abductions should be handled.

If they were taken by the Shia al-Mahdi Army — the most likely scenario — negotiations might be possible. However the killing last week in Basra of Abu Qadir, the leader of al-Mahdi Army in the city, may have made any prospective talks more difficult. Military authorities in Iraq have been on alert for retaliation since Abu Qadir was shot by Iraqi special forces in a mission supported by British troops.

If, on the other hand, the abductions were the work of any number of shadowy Sunni groups, such as al-Qaeda in Iraq, there is virtually no one to negotiate with. Such groups are believed to have been responsible for the killing of most Western captives.

“We don’t yet know if the kidnappings were linked to the Basra killing,” one Whitehall source said.

However, Abu Hussein, a commander of al-Mahdi Army in Basra, was quick to claim responsibility: “It is not only a reaction but it is the end of the British here. We will take revenge on the British. It is not just this operation but there will be more and bigger operations against them. In quality and quantity.”

Al-Mahdi Army commanders in Baghdad, however, denied the milita had anything to do with the kidnappings. “We called all our groups immediately afterwards and no one said they did it,” a senior al-Mahdi Army commander in the Iraqi capital told The Times.

“We called the Iraqi police in the area and they told us it was uniformed men speaking with Sunni accents,” he said.

The Britons were seized from a Finance Ministry building on Palestine Street after scores of men in police commando fatigues arrived in vehicles and sealed off the area. The police force is believed to have been heavily infiltrated by al-Mahdi Army, which is loyal to the nationalist cleric Hojatoleslam Moqtada al-Sadr.

A crisis team, which includes diplomats, police hostage negotiators and intelligence specialists, was formed to try to establish lines of communication with the kidnappers.

The captives were a consultant working for BearingPoint, an international firm providing technical and computer advice to the Iraqi Government, and four security guards employed by GardaWorld, a Canadian-owned company which, for its Middle East contracts, has offices in London and Hereford — the home of the SAS.

Last night Joe Gavaghan, a spokesman for the company, said: “We are notifying all the families of the hostages and we’re trying to determine exactly what happened.”

British diplomats held urgent talks with the Iraqi authorities to establish the sequence of events that led to the snatch which happened at 11.40am local time at a building located outside the heavily protected international green zone.

One Western security adviser said: “The British Embassy has gone into shutdown. It’s not looking good.”

The Cobra meeting, chaired by an official, not a minister, compared the latest abductions with the three other cases when Britons were taken hostage in Iraq: Norman Kember, 76, who was rescued by British special forces last year , and Kenneth Bigley, 62, and Margaret Hassan, 59, who were both murdered.

The kidnapping provoked an immediate security lockdown in Baghdad. Iraqi police reinforced roadblocks and questioned drivers for information, while Iraqi army tanks scanned passing traffic.

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