At least three have been killed in combat, including one whose role in an Iraq suicide bombing in February was disclosed by police only last week.

The growing problem of militants from Britain travelling to Iraq has been highlighted by Eliza Manningham-Buller, director-general of MI5, in recent briefings to Tony Blair.

The MI5 boss warned the prime minister that would-be suicide bombers and other fighters who want to kill British and American troops were using “a number of routes” to get to Iraq from Britain. Most have travelled to Damascus in Syria before being smuggled over the border to meet rebel leaders.

Senior police sources say that they are trying to unravel a British-based network recruiting “martyrs” for the Islamic “holy war” in Iraq. The operation emerged following the arrest last week of a 40-year-old man who is alleged to have shared a house in Manchester with Idris Bazis, a 41-year-old French Algerian who blew himself up during a terrorist attack in Iraq four months ago.

Anti-terrorist detectives believe that Bazis travelled from Britain to Iraq in January after Abu Musab al-Zarqawi, the Al-Qaeda leader in Iraq, appealed for volunteers to carry out suicide attacks in a bid to undermine the election.

“We shouldn’t be surprised that there are people travelling from the UK to Iraq to fight or try to fight,” a security official said. “You’re looking at dozens of people in total coming from the UK since the war.”

The official said that the recruits were mainly young Muslim men in their twenties and thirties who had volunteered independently of each other. They ranged across the social classes.

Some, like Richard Reid, the shoe-bomber from Brixton, south London, converted to Islamic fundamentalism after a life of petty crime. Others, such as Saajid Badat, were from middle-class backgrounds. He was jailed in April for planning a similar attack.

Some have been recruited by radical imams in mosques. Others have been lured to the cause by appeals on extremist websites. They include Britons and nationals from other countries, including north Africa, who had settled in Britain. Although an estimated 70 had gone in the past two years, few had returned, the source said.

Larger numbers of terrorist recruits left Britain during the late 1990s to fight the “holy war” in Bosnia, Chechnya and Afghanistan. The number of Britons estimated to have travelled to terror training camps in Afghanistan before 2002 has been put as high as 3,000 by MI5.

According to a senior rebel commander in Iraq, at least two Britons have been killed in the fighting there in the past year. One was named as Abu Hareth, 22. He is said to have joined an Al-Qaeda unit, only to be killed during a battle with American troops in the town of Taji, 20 miles north of Baghdad.

The rebel commander, who used the name Abu Ahmad, said that another Briton, called Ammar and also 22, had been killed during clashes between Al-Qaeda and American troops around the time of the Falluja offensive in April last year. Ammar had travelled to Iraq with his brother Yassr, 18, shortly after the fall of Baghdad in March 2003. “They could not wait to go out and fight and kept on asking when they would go into battle,” the commander said.

He said that Al-Qaeda had “branches, supporters and financiers” in Britain as well as in France, Spain and Germany.

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