The head of a British parliamentary counter-terrorism committee called on Saturday for the government to clarity whether Al Qaida operatives had tried to infiltrate the MI5 domestic security agency.
Patrick Mercer, an opposition Conservative and chairman of parliament's sub-committee on counter-terrorism, said he had been told up to six Muslim recruits had been ejected from the spy agency because of concerns about their backgrounds.
Two of the men who had attempted to join MI5 after the July 7, 2005, London suicide bombings had attended training camps in Pakistan while the others had unexplained gaps in their curricula vitae, Mercer told the Daily Telegraph newspaper.
"I would be very, very surprised had our enemies had not tried this but I would like some clarification from the government to see how successful we've been at detecting them," he told Sky News.
Mercer said he wanted British Home Secretary (interior minister) Alan Johnson to detail how far down the recruitment process the men had got before they were weeded out, and to ensure the vetting process was as tight as possible.
"What concerns me is not all these individuals ... have necessarily been nailed," he told BBC TV.
The Home Office made no comment about Mercer's allegations, however an unnamed senior security source told the Telegraph no one had been asked to leave the spy agency after starting work or training because they had been linked to extremists.
MI5 significantly increased in size after the 2005 bombings by four young British Islamists, a process Mercer said should have started much sooner after the Sept 11 US attacks.
He said the recruitment rush had given Al Qaida an opportunity, although the fact that militant sympathisers had been tracked down suggested vetting procedures were good.
"Of course they (MI5) should be attracting recruits from all sorts of backgrounds," he told Sky. "The fact remains it is not without risk. A subversive organisation worth its salt will of course try to infiltrate."
Last month Britain downgraded the threat from international terrorism to "substantial", the third-highest on a five point scale.
The alert system had never previously fallen below the second level of "severe" since its introduction in August 2006. The government said despite the change the threat from groups such as Al Qaida remained "real and serious".