Abdul Rahman had been a constable for almost three years when MI5 warned that he might have visited a training camp in Pakistan when he travelled there. He resigned rather than be dismissed from the force and is now suing Scotland Yard for compensation. He says he is entirely innocent and has never been to a terrorist training camp.

Mr Rahman, 33, is the first British policeman ever disclosed to have failed counter-terrorism checks.

Scotland Yard submitted in legal documents that it acted against Mr Rahman “for the purpose of safeguarding national and public security”. A source familiar with the case said there were either one or two other officers who had also lost their jobs because of MI5’s suspicion that they might have trained as terrorists. “There was concern that these people had come into the force under false pretences,” the senior Metropolitan Police source said. “There were two or three cases at the same time that were of a similar nature, where there were concerns about potential terrorist links.”

The fact that he was under MI5 suspicion was disclosed in court documents made public as he fights a lengthy legal case over his departure from the force.

The case is so sensitive that it is being heard by a security-vetted judge.

Mr Rahman, a Muslim who was born in Bangladesh before being raised in London and becoming a British citizen, does not dispute that he went to Pakistan in 2001.

Mr Rahman became a probationary constable in the Metropolitan Police in September 2003 then attended Hendon Police College, completing his initial training in March 2004. As part of his recruitment Mr Rahman underwent a process of security vetting known as a counter-terrorist check (CTC). However, his security clearance was suspended on June 22, 2006.

Shortly before Mr Rahman’s clearance was suspended, MI5 rechecked details of officers and civilian staff at the Met, and other forces, against their records of suspects who had been to Pakistan or Afghanistan and who it suspected might have attended terrorist training camps or madrassas — Islamic schools — run by extremists. Mr Rahman, who is married with four children, declined to comment on the case. He claims he is the victim of racial and religious discrimination.

Mr Rahman’s father, who lives in Poplar, east London, said: “He is a very genuine and very honest man. He has nothing to hide. He is a family man. The allegations against him are untrue. He is very pious, he prays five times a day and I think he didn’t fit in in the police because of that.”

Last month, after a five-year legal battle, the Employment Appeal Tribunal ruled that his case could be held in secret although Mr Rahman had wanted a public hearing. Mr Justice Mitting, a High Court judge who also specialises in terror cases in his role as chairman of the Special Immigration Appeals Commission, ruled that Mr Rahman and his legal team would be banned from parts of the hearing that concerned issues of national security.

Scotland Yard applied to have Mr Rahman’s case heard in secret because it is keen to protect intelligence sources which provide highly-sensitive information.

Scotland Yard’s vetting unit is regarded as one of the best in Britain, mainly because the force has countrywide responsibilities in counter-terrorism. However, it is understood that there are difficulties carrying out full checks on applicants born abroad or who have spent a long time living outside Britain.

MI5 carries out the CTC vetting on behalf of the Met, other police forces and government departments.

It is the lowest of three levels of vetting under the Cabinet Office’s Security Policy Framework. However, individuals who pass the CTC procedure are allowed access to documents classified as “confidential” and can be granted occasional access to “secret” documents. They also have access to areas where classified papers are stored. It can take up to six months to complete vetting and clearance is valid for three years.

“If police officers lose their CTC they are effectively unemployable,” said a police source. “They cannot be allowed to access the Police National Computer, or other vital things in the course of their day-to-day work.”