Data matching doubled to almost 11,000 the official estimate of illegal immigrants cleared to work in security posts, according to the home secretary.

Jacqui Smith told MPs yesterday that she ordered the process be used to speed up the production of accurate figures on the size of the security problem. And the technology proved the situation to be twice as bad as the Home Office's initial estimates, produced last month.

"I ordered automatic matching between the Security Industry Authority (SIA) list of non-European Economic Area (ERA) nationals, and databases operated by the Border and Immigration Agency and UK Visas," said Smith.

The scandal was originally kept under wraps for at least five months, in order, said the home secretary, to make sure the information was correct.

Initial checks on the 40,000 non-ERA nationals licensed before 2 July, revealed the government was "fully satisfied" that 29,000 are entitled to work, "believed" 6,700 do not, and is not satisfied the remaining 4,400 have proved their right to work.

Smith insisted that although data checks are possible, it is the employer's duty to ensure staff have the right to work in the UK, not that of the government. But Liberal Democrat spokesman Nick Clegg described the employment of illegal workers by the Metropolitan Police as another example of "Keystone Cops incompetence".

The SIA has written to all companies on its database to order them to check registers to ensure their staff have the right to work. The site is receiving more than 1,000 hits a day, according to Smith.