The Home Office was embarrassed further last night after secret memos emerged illustrating their attempts to cover-up immigration “blunders”.

The documents revealed that Jacqui Smith, the Home Secretary, had been warned four months ago that thousands of illegal immigrants had been cleared by the Security and Industry Authority (SIA) to work in Whitehall.

Officials recommended a news blackout on the revelation amid fears it would generate negative headlines.

The “blunders” only emerged publically on Sunday when it was revealed that one illegal immigrant had been sent to guard Gordon Brown’s car while another eleven were working at Scotland Yard.

The Home Office then confirmed that at least 5,000 illegal immigrants had been cleared to work by the SIA. But memos circulated within the department show that she was aware of the problem from at least July 12.

One memo, sent from Ms Smith’s private secretary to other Home Office ministers and senior officials on August 9, said that the Home Secretary did not think the department had explanations for the blunders which were “good enough for press office or ministers to use to explain the situation”.

Another memo from a civil servant to ministers, dated August 20, says that because the full scale of the problem is still not known, no public announcement should be made. The memo adds: “Any announcement about illegal migrant workers and SIA licences would not be presented by the media as a positive story. Press Office has defensive lines to take . . . to use reactively in the event that this story leaks to the media.”

A further memo ten days later said that if the media discovered that thousands of licences had been issued to illegal workers, some of whom were working in high-profile security jobs, there would be significant criticism.

The memo, sent by an official to Ms Smith and her colleagues Tony McNulty, Liam Byrne, Vernon Coaker and Lord West of Spithead and which has been seen by the Daily Mail, states: “They [media] are likely to refer back to other Home Office so-called blunders which adversely affected coverage of the department last year and this.”

Opposition politicians last night criticised the Home Secretary’s actions when she was first told of the failings.

Damian Green, the Shadow Immigration Minister, said: “The Home Secretary has been caught red-handed putting the short-term interests of the Government before the long-term task of solving a real problem. Why if she knew about this in July has she not told Parliament anything about it? One of the memos uses the phrase “should this issue ever come to light”. Was the Home Secretary hoping she would never have to tell us about it?”

The Tories called for an investigation into what they called the latest evidence of incompetence at the Home Office, which was famously branded “not fit for purpose” by John Reid, Ms Smith’s predecessor.

A Home Office spokesman said at the weekend that the SIA took “immediate action” as soon as it had become aware that some licence holders were being employed illegally.

Times Recommends

• 5 Christmas romance confessions
• Has champagne lost its sparkle?
• Your tailor-made Christmas