MI5 is facing allegations that it mistakenly recruited Al Qaeda sympathizers who were trying to infiltrate the British secret service.
Patrick Mercer, a Conservative member of Parliament, is demanding a probe into claims six Muslims were thrown out of MI5 because of concerns about their past.
Two of the six allegedly attended Al Qaeda training camps in Pakistan while the others had unexplained gaps of up to three months in their resumes.
Mercer, counter-terrorism sub-committee chairman, told Sky News that MI5 is believed to have detected two sympathizers at a "fairly early stage" of their secret service training.
The others allegedly did not make it through vetting procedures.
However, Mercer wants clarification on how successful the security services have been at detecting enemy infiltrators.
"This is exactly the sort of work that MI5, MI6 and the other security services should be up to," he said. "Of course, they should be attracting recruits from all sorts of different backgrounds, diverse backgrounds. The fact remains, it is not without risks... a subversive organization that is worth its salt will, of course, try to infiltrate."
A government spokesperson said the service "takes vetting very seriously."
"All candidates are required to undergo the most comprehensive process of security vetting in the UK."