The Home Office is embroiled in a fresh scandal after admitting that illegal immigrants had been cleared for jobs as security staff.
Ministers have ordered new checks to be carried out on hundreds of thousands of people vetted by the Security Industry Authority (SIA) over the past three years.
The SIA had not been ensuring that applicants were entitled to work in the UK before granting licences over the past three years, according to the department.
Investigators are reported to have already discovered illegal immigrants working at the Metropolitan Police, in Whitehall, and at ports and airports since the situation came to light in July.
The Sunday Mirror claimed that 5,000 illegal immigrants were estimated to have been employed in posts such as security guards and bouncers.
Shadow immigration minister Damian Green said the news was "extraordinary" when the Home Office had previously discovered it was employing illegal immigrants as cleaners. "From these new revelations it looks like no effective action was taken to check who has access to some of the most sensitive buildings in this country."
Lib Dem home affairs spokesman Nick Clegg said public confidence in Whitehall competence was being "hammered" again.
A Home Office spokesman said the SIA had taken "immediate action" as soon as they had become aware that some licence holders were being employed illegally.
Cabinet Office Minister Ed Miliband was asked about the controversy on BBC1's Politics Show .
"(The SIA) introduced a new licensing system in July which instituted the kind of checks we need on people to make sure they can be legally working in this country," he said. "Now they are going to check existing employees from outside the European Union where there is a risk of them being employed illegally. This is a responsibility on employers. Employers should not be employing people illegally."
