A member of security staff at the Home Office was arrested on Friday after being exposed as an illegal immigrant.
The blunder was uncovered after the Border and Immigration Agency (BIA) identified "problems" with a sub-contracting firm used by the department.
Replying to a question tabled in Parliament, Home Secretary Jacqui Smith said: "The Home Office is now working with the sub-contractor to recheck individually the right to work of all their security staff supplied to the Home Office.
"Furthermore, the Permanent Secretary has taken immediate steps to tighten the procedures for checking the identity and immigration status of those working in the Home Office, whether as a civil servant, employed by a contractor, or in any other capacity."
A Home Office spokesman confirmed the individual had been granted a licence by the Security Industry Authority (SIA), and was now subject to deportation proceedings.
The illegal worker was identified and arrested the day after Ms Smith told the Commons that more than 11,000 could have been cleared by the SIA to take on sensitive security jobs since 2004.
The authority had been failing to check whether applicants had the right to work in the UK before handing out licences.
As a result, illegal immigrants found employment on sensitive Whitehall sites and with the Metropolitan Police.
Shadow home secretary David Davis said: "Last week the Home Secretary could not go out of her way enough to lay the blame for the SIA shambles at the door of employers.
If she is going to try and avoid responsibility in such away she should at least check her own house is in order. Who will the Home Office now prosecute and fine? Itself?"
Read More
MORE ABOUT