Mercer said he wanted Alan Johnson, the British home secretary [interior minister], 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.
He said: "... I do need reassurances from the home secretary that none of these individuals has actually been successful in becoming fully fledged paid-up members of one or another intelligence agency."
The UK's secret service significantly increased its size following the 2005 bombings in London carried out by four young British citizens.
|Mercer wants the UK government to say how successful it has been at detecting infiltration
Mercer said a recruitment rush had given al-Qaeda an opportunity to inflitrate the organisation.
"Any subversive organisation worth its salt will try to penetrate its opponent," he said. "Heavens above we're trying to penetrate al-Qaeda.
"My regret is that the recruiting process did not start earlier - probably in September 2001, after the bombings in America - and therefore perhaps it could have been conducted in a rather more measured faction."
A spokesman for Britain's home office said: "MI5 takes vetting very seriously indeed. All candidates are required to undergo the most comprehensive process of security vetting in the UK.
"Recruits go through extensive vetting and it is not unusual for a number to drop out or fail at the earliest stages for a variety of reasons."
Last month, the UK 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 that despite the change, the threat from groups such as al-Qaeda remained "real and serious".