Patrick Mercer, chairman of the Home Affairs counter-terror sub-committee, said he was told six recruits were ejected after worries about their past.
Two allegedly attended al-Qaeda training camps while the others had unexplained gaps in their CVs, Mr Mercer told the Daily Telegraph.
A Home Office spokeswoman declined to comment on the claims.
Mr Mercer said he had learned that MI5 had dismissed the six recruits some time between 2005 and 2007.
The MP said he feared that, in the aftermath of the bombings on London's transport network in July 2005, the security services had rushed to try and take on Muslim recruits, and that had potentially allowed al-Qaeda sympathisers to infiltrate the security service.
He has written to the Home Secretary Alan Johnson asking for further details.
Mr Mercer said the Commons Home Affairs committee may investigate the issue next month.
He told the newspaper that the government should have been prompted to expand the security services following the attacks on New York on 11 September 2001.
"In fact it took an attack on this country for such measures to be started," he added.
"But at this point it was an unseemly rush of which our enemies, not unsurprisingly, took advantage."