About 1,500 more foreign prisoners are being held in English and Welsh jails than was thought, it has emerged.
The home secretary offered to quit
The Home Office has confirmed there are 915 inmates whose nationality has not been determined. Another 600 prisoners falsely claimed to be British.
Charles Clarke is under fire after admitting 1,023 prisoners facing potential deportation were set free.
It was only on Tuesday that police were asked to find the missing men, despite concerns being raised last July.
The Home Office admission that more than 900 prisoners had no nationality recorded means the number of foreigners behind bars in England and Wales could be higher than the 10,000 reported.
A spokesman said: "At the end of February 2006 there were 915 prisoners in prison establishments in England and Wales with no nationality recorded on the central IT system."
Progress report pledge
On Tuesday Charles Clarke told Tony Blair that he was prepared to quit but the prime minister turned down the resignation offer.
It has subsequently become clear that Mr Blair did not know then that 288 cases of prisoners - including rapists, paedophiles and murderers - had been released since ministers had been alerted to the problem.
Mr Clarke has pledged a progress report on tracking down the missing ex-prisoners.
Police meanwhile have begun processing hundreds of names and returned their first results to the Home Office.
Warning in 2001
The Association of Chief Police Officers (Acpo) said a team at its London HQ was running police national computer checks (PNC) on names being provided by the government department.
The checks will show if police have current addresses for the missing 900 criminals.
An Acpo spokeswoman said: "We are being given names and as and when we're getting them, we will do a PNC check.
"It began yesterday. I am unable to say how many we have done so far."
Ex-chief inspector of prisons, Lord Ramsbotham, said he had warned ministers back in 2001 that the systems for dealing with growing numbers of foreign prisoners were unsatisfactory.
In his annual report to Mr Clarke's predecessor, David Blunkett, Lord Ramsbotham recommended the appointment of an official to be responsible for foreign nationals.
Lord Ramsbotham told BBC News on Wednesday the Home Office ought to be broken up into separate ministries for homeland security and justice - the two areas together were "far too big for anyone to manage".