The policeman — who firmly denies the allegations — is said by police sources to be one of three Muslim officers questioned in a Yard search for terror “sleeper cells” in its ranks.

The move follows disclosures earlier this month that Islamic terrorist sympathisers had attempted to infiltrate the intelligence services by applying for jobs in MI5.

It raises the risk of terrorist “fifth columnists” passing on information about secret operations and compromising the identities of undercover agents.

The Metropolitan police last week said that it would look at individual cases if there were concerns, but added that it was not reassessing the backgrounds of all its 400-500 Muslim staff.

The officer, who is in his mid-twenties, was placed on restricted duties at a London police station earlier this month. Police sources say his security clearance for sensitive databases has been withdrawn pending the results of an internal inquiry.

The inquiry appears to have been triggered by the belated discovery that the officer made a trip to Pakistan before joining the police in 2001. He returned to Britain shortly after the September 11 terrorist attacks.

During an interview with the Yard’s secret vetting unit, which works closely with MI5, it was suggested he may have attended or associated with people at an Al-Qaeda training camp. The British-born officer, whose parents are from Pakistan, denies knowingly meeting terrorists.

He is understood to have told his interviewers that he went abroad to study and this involved briefly attending a madrasah — religious school — as a way of getting into technical college in the UK. He abandoned his studies to join the police in 2003.

Some madrasahs in Pakistan have a reputation for attracting extremists and radicalising young Muslims. Shehzad Tanweer, the London suicide bomber, is said to have visited one in the months before last year’s attacks.

The Association of Muslim Police (AMP) says there are two other investigations in which Muslim officers have been interviewed about trips to Pakistan.

Tahir Butt, secretary of the AMP, has raised the cases with senior management. He said that a policy which targeted Muslims and not other religions would be “wholly inappropriate”.

Explore UK News

• Crime News

• Education News

• Health News

• Science News

• Scotland News

Times Recommends

• Forces chief faces new battle - the economy
• Seven days that shook the world
• Icelanders wake up and smell ... the fish