THE government has secretly calculated there are about 500,000 illegal immigrants in Britain despite repeated claims by ministers that they do not know the scale of the problem.

The figure has been compiled by Home Office officials. Yet one of its ministers told MPs in February there was “no official estimate”.

The research was ordered by Tony Blair more than a year ago “as a matter of urgency” following a Downing Street summit on immigration, a confidential Whitehall memo reveals.

However, in the face of a political controversy over lax controls at Britain’s borders, experts involved were told not to reveal the figure. It includes not only migrants who have illegally entered Britain to work in the black market but also failed asylum seekers who should have been deported.

The estimate — equivalent to the population of Sheffield — is far higher than previous figures from campaigners such as Migration Watch UK and is likely to intensify the row over immigration.

Last week the Tories claimed immigration controls were a shambles after an illegal immigrant was convicted of murdering a policeman and plotting a terrorist attack with the poison ricin.

David Davis, the shadow home secretary, said: “This government now admits that the number of illegal immigrants is at least 500,000 and it could be much more. But yet again it is covering up the truth from the public. This smacks of a desperate attempt to conceal its own facts.”

The existence of the estimate was confirmed this weekend by Professor John Salt, director of the Migration Research Unit at University College London, who was commissioned by the Home Office to study the number of illegal migrants.

“I have seen the first run through (of the figures),” he said. “I was consulted as an expert and I made some comments.”

Salt’s disclosure appears to contradict statements by Des Browne, the immigration minister, who told MPs in February there was “no official estimate for the number of illegal immigrants working in the United Kingdom”.

Only last week Charles Clarke, the home secretary, told the BBC’s Newsnight programme that he did not know how many illegal immigrants there were in Britain.

Salt, a world expert on immigration, said his estimate — provided to the Home Office — was between 450,000 and 500,000 illegal immigrants. “If I were in a court of law and asked what was my best estimate, that’s what I would say,” he said.

His paper had also originally included detailed estimates of the numbers of illegal migrants working here, but he said the figures were removed by the Home Office when it was published last year.

When he was given a copy of the department’s estimate of the number of illegal immigrants last October, he said officials had told him they did not want to release the information and had asked him to keep the figure confidential. “If they don’t want to release the information, I can’t release it,” he said this weekend.

However, a Home office source confirmed the department’s figure was similar to Salt’s estimate of 500,000. According to the confidential Downing Street memo, the research was ordered by Blair when he and Home Office ministers held an “asylum stocktake” at No 10 in March last year.

The leaked memo states: “The prime minister said there were some necessary first steps that should be undertaken as a matter of urgency . . . A dedicated team was needed . . . to compile a robust fact base on which ministers could rely.

“This should include data on who was in the country, by nationality . . . as well as an estimate of illegal migrants and their methods of entry. ”

Salt said the 450,000- 500,000 figure was based on a study of amnesties given to illegal migrants in more than a dozen countries. It was calculated by dividing the number of legal migrant workers by the number given an amnesty. The percentage could then be applied to Britain where, the 2004 Labour Force Survey says there are 2.85m foreigners working legally. If the proportion was 20%, the number of illegal foreign workers would be 570,000. But Salt said he preferred a lower ratio.

He said the number did not include spouses, dependants and those not in work. Experts say if these were taken into account, the final figure could be nearer 1m.

A senior aide to Clarke said: “There are no official estimates and, as Michael Howard has said, by its very nature, illegal immigration is very difficult to measure and any estimates would be highly speculative.”

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