Details of more than 20 of the 80 most dangerous foreign prisoners freed without facing deportation cannot be found on the national police computer.
Charles Clarke has apologised after the prisoners walked free
They include one murderer and a rapist. A source told the BBC police may not have sufficient details to run a check.
But Home Office minister Hazel Blears insisted the most serious offenders would be out on licence and monitored.
Home Secretary Charles Clarke is to report on progress tracing hundreds of the foreign convicts freed since 1999.
He is to give a written update to the House of Commons speaker on Friday afternoon outlining what has happened to some of the 1,023 foreign prisoners released between 1999 and last month.
His statement comes as an opinion poll for the BBC's Daily Politics suggests that 63% of voters believe he should be sacked as home secretary.
Ms Blears said she was unable to say whether all the offenders were registered on the national police database or not, but added that every offender would have been registered by the Prison Service.
"Things don't just have to be on the police national computer," she told BBC Two's Newsnight.
"Clearly the Prison Service will have a record of who they were, when they came in, what their sentence was, and all those sources of information will be being used to make sure that they're being tracked properly."
She said the foreign inmates would have been dealt with in exactly the same way as British ones.
"What I can say to you is that the police are going through every single one of these individuals," she said.
"The people who've committed serious offences will be on licence and will be being monitored and the public should be reassured about that."
Newsnight reported that the Association of Chief Police Officers (Acpo) has appealed to regional police forces to search their own databases for the 20 foreign nationals who could not be found on the national system.
An Acpo spokeswoman said the association was confident all 80 of the most dangerous criminals were on the database.
The problem was that 22 could not be found in the computer records because the Home Office had not supplied enough information to identify them.
For some of the former inmates, only a surname had been given.
The Police National Computer stores details of convictions and cautions.
Probation records are thought to have provided the best guide to the criminals' whereabouts, because many are under supervision or on licence.
Immigration files have also been reviewed.
The searches come amid mounting pressure from the Conservatives and Liberal Democrats for Mr Clarke to resign over the issue.
Shadow home secretary David Davis accused Mr Clarke, who has apologised, of a "major failure" and said he "had to go".
"Charles knew about this from July of last year - 10 months ago, his assertion - and we only got to the point of doing anything about it... two weeks ago," he told BBC One's Question Time.
Prime Minister Tony Blair robustly defended Mr Clarke and two of his Cabinet colleagues who have been under pressure in recent days.
He laughed off claims that he had suffered his own "Black Wednesday" the day before.
He was speaking after nurses booed Health Secretary Patricia Hewitt at a nurses' conference over job cuts, Deputy Prime Minister John Prescott admitted to having had an affair.
* The Daily Politics opinion poll was carried out by Populus, and involved interviewing 1004 adults aged over 18 by telephone on 26 and 27 April.
In addition to 63% of voters believing Mr Clarke should be sacked, it also found that 65% of voters believe "being home secretary is a very difficult job and no one will ever get it right".
It also found 84% of people disagreed with the statement "the Government has immigration under control" and 67% disagreed with the statement that Tony Blair's government has made Britain "a safer place to live by tackling crime and anti-social behaviour".