LONDON, England (CNN) -- The British government on Saturday responded to a senior lawmaker's call for an investigation into whether its intelligence agency, MI5, inadvertently recruited individuals involved with terrorist activity.
Conservative MP Patrick Mercer, seen here in 2007, says he based his allegations on discussions with sources.
A Home Office spokesman told CNN the department is not aware of MI5 recruiting extremists of any kind.
"MI5 takes vetting very seriously indeed," the spokesman said. "All candidates are required to undergo the most comprehensive process of security vetting in the UK."
Patrick Mercer, a conservative member of Parliament, told British media that according to his discussions with multiple sources over the past six months, as many as six suspicious individuals started the recruitment process with MI5.
He said the alleged breach occurred when MI5 tried to beef up intelligence in the wake of the July 2005 bombing on London's transit system that killed 52 people.
"I have no doubt that a couple of these individuals had been trained by subversive elements. Now whether they were al Qaeda or others, I don't know," Mercer told British television network ITN.
"But it would seem to be clear that they certainly had been abroad and they certainly had paramilitary training of some sort."
Mercer, chairman of the counter-terrorism subcommittee, said two of the individuals got "some way" into their MI5 training before it was discovered that they had been "directly or indirectly involved with terrorist activity, probably in Afghanistan or Pakistan."
Another three or four individuals had "black holes" in their background and were kicked out before they began MI5 training.
The Home Office spokesman said applicants are extensively vetted and that it is not unusual for "a number to drop out or fail at the earliest stages for a variety of reasons."
Mercer said he is requesting that the Home Office investigate the allegations and reassure him that "no successful penetration has gone on" at the intelligence agency.