Mounting evidence emerged yesterday that an Iranian-backed Shia Muslim militia was behind the abduction of the five Britons.

According to Iraqi and British officials working on the case, the kidnapping of a British IT expert and four private security guards bore all the hall-marks of al-Mahdi Army, the militant group headed by Hojatoleslam Moqtada al-Sadr.

The Shia religious leader returned to Iraq last week and there have been fears that he would order a new wave of attacks against American and British targets.

The organisation was said to have had the motive to take the Britons and the opportunity to conduct the abduction and get away, as well as a previous record of similar operations.

Al-Mahdi Army is the largest militia in Iraq and draws its support from the Shia underclass across the southern and central areas of the country. The group is blamed for mounting repeated attacks against American and British forces in Iraq, using deadly explosives supplied by Iran. It is also known to have infiltrated the Iraqi security forces, in particular the police. Abdul Mahdi al-Mutiri, a senior official in the Sadr movement, denied that al-Mahdi Army was responsible, but American, British and Iraqi officials were still working on that assumption yesterday.

Witnesses said that the 40-strong kidnap group included several men dressed in the uniforms of the Iraqi police commandos. They were also travelling in vehicles normally used by the police.

“It may be the Mahdi Army because the location [of the abduction] is in their field of operations,” Hoshyar Zebari, the Foreign Minister, said. “It has been known for some time that the Interior Ministry police, security units and forces are corrupt, are penetrated,” he added.

Al-Mahdi Army’s powerbase is Sadr City, a sprawling slum in eastern Baghdad very near the site of Tuesday’s abduction at a Finance Ministry building on Palestine Street. The Finance Minister is Bayan Jabr, a senior official in a rival Shia movement, the Supreme Council for the Islamic Revolution in Iraq.

Mr Jabr formerly commanded the group’s military wing, the Badr Brigades. He was removed as Interior Minister after allegations that he was running Shia death squads and had infiltrated his fighters into the police force.

British officials said that if the ministry had been under the control of Iraqi Sunni Muslim or Kurdish guards “the kidnappers would never have got through the gate”. That it was guarded by Shias meant that the abductors could get in, seize the Britons and leave without a shot being fired.

If al-Mahdi Army is responsible it could mean that the hostages will eventually be set free. The Sadr movement has several MPs in parliament and until recently held ministerial portfolios. Even if it is hostile to the presence of American and British troops it does cooperate with the Government when it needs to.

Sunni insurgent groups, such as al-Qaeda in Iraq and its allies, usually execute their Western captives.

There are two possible motives for the kidnapping. One is revenge for the killing by British troops last week of Wissam al-Waili, the Mahdi Army leader in Basra, who was shot in a gun battle. Canon Andrew White, a vicar based in Baghdad who knew the kidnapped men, said that there was a “strong possibility” of a connection between the two incidents.

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