Labour has denied claims of a Government cover-up over the true scale of illegal immigration to Britain.
It followed weekend reports that Home Office officials have secretly calculated that there are 500,000 illegal immigrants in the UK.
The Government has always said that it does not know how many illegal immigrant there are in the country.
The figure was said to have been confirmed by Professor John Salt, the director of the Migration Research Unit at University College, London, who was commissioned to study the issue by the Home Office.
Shadow home secretary David Davis said that Home Secretary Charles Clarke must explain the Government cover-up on immigration.
“It has become clear this weekend that the Home Office has calculated that there are around 500,000 illegal immigrants living in Britain today,” Mr Davis said.
“Labour ministers have repeatedly maintained that there is no Government estimate about the scale of the problem of illegal immigration.
“Why has Mr Blair’s government said that an estimate does not exist when we now know that it does?”
However, a Labour Party spokesman said that the Government had no figures for illegal immigration.
“There is no official estimate, published or unpublished, of the numbers of illegal immigrants,” the spokesman said.
“As Michael Howard said when he was Home Secretary: ‘There are no official estimates of illegal immigrants into the UK. By its very nature, illegal immigration is difficult to measure and any estimates would be highly speculative’.”