Five Al-Qaeda suspects are believed to have trained as part-time soldiers with the TA. At least one is now in custody.

The Ministry of Defence (MoD) has confirmed that other terror suspects have attempted to join the TA, but says they were rejected after undergoing security checks.

The connection with Britain’s Al-Qaeda network was uncovered in a series of wide-ranging investigations by MI5 and Scotland Yard’s Special Branch.

It is believed the terrorist suspects may have been taking advantage of military training as well as gaining access to bases and weapons.

Patrick Mercer, the Tory homeland security spokesman and a former army officer, said Al-Qaeda terrorists could use TA passes to penetrate security at key MoD establishments such as the permanent joint headquarters at Northwood in Greater London.

“This could have very serious security implications. Clearly in the war against terror you need to know who your friends are. The last thing we want is the enemy masquerading as our own people,” Mercer said.

The TA has about 41,000 members and comprises a substantial portion of the 102,000-strong British Army and has 329 centres throughout the country.

Most recruits have access to small arms and undergo infantry training. Security experts say while such access may be limited, terrorists could get greater benefit from targeting selected units such the Intelligence Corps, the Royal Engineers and logistics units.

The TA’s Royal Engineers would give them access to explosives. The Royal Logistics Corps would provide access to military fuel dumps and a fleet of tankers. Many army tanker drivers now on duty in Iraq are TA soldiers.More than 7,000 have been called up to serve in Iraq — the largest deployment in its 97-year history.

The MoD says all TA applicants went through the same “security check” procedure as members of the armed forces.

Candidates fill in a standard form that is passed to the Defence Vetting Agency, which carries out checks on the applicant’s nationality and family background.

Further checks are made against criminal records and candidates are asked to disclose if they have any known terrorist affiliations.

“They are very basic,” said an MoD spokesman. “We do check the information on the form and issues are spotted and managed as appropriate. People do fail the vetting because of associations with terrorist organisations.”

Details of the TA connection have emerged in an inquiry by Scotland Yard into the background of several suspects arrested in Britain earlier this year. The arrests followed a tip-off from Pakistan, although details of the case cannot be reported for legal reasons.

Senior police and intelligence officers want to know whether terrorists are making a concerted effort to penetrate the TA as part of an initiative to gather intelligence for an attack.

Some believe they are attracted to the TA because it gives them a legitimate cover to undergo military training.

The territorials have previously been the target of terrorist interests. In the mid-1990s, Special Branch uncovered a plan by hardline animal rights activists to acquire explosives from weapons dumps.

MI5 and Scotland Yard has known for at least two years that as well as targeting the TA, Al-Qaeda recruiting sergeants in mosques are sending potential recruits on outward-bound courses to improve their fitness and undergo mock weapons training.

In the early 1990s the IRA bombed several TA barracks including the headquarters of the Honourable Artillery Company in the City of London, and a barracks in Hammersmith.

Despite a series of arrests by police and MI5, security officials stress that Al-Qaeda still poses a serious threat to Britain. Recently acquired intelligence suggests the group has been planning a “spectacular” attack, possibly using car bombs against high-rise office blocks in the City of London.

There are also fears that the group could target shopping centres and railway stations.

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