The Conservatives have said the position of two Home Office ministers would be "untenable" if reports that they knew Britons convicted abroad were not being tracked were true.
It was revealed last night that Home Office minister Tony McNulty and junior minister Joan Ryan knew that files of 27,000 offenders had not been properly entered onto UK police databases - and therefore were unknown to British authorities - last October.
Home secretary John Reid only found out about the row earlier this week when a member of the Association for Chief Police Officers (Acpo) reported to a select committee. Acpo discovered the files when it took responsibility for the database last May.
Yesterday Mr Reid told the House of Commons that if he had known about the problem, he would have included it in his review of the Home Office launched last spring in the wake of his assertion that the department was "not fit for purpose".
However, last night shadow home secretary David Davis said that if reports that others knew about the problem were true, "the position of two ministers would be untenable". He demanded Acpo publish the letter it sent to the Home Office, including all replies.
Mr Reid has launched an internal inquiry into the problem, including why he was not informed of it earlier. But Mr Davis demanded that this inquiry must be led by a judge, not a civil servant, to ensure it properly established the facts.
Liberal Democrat home affairs spokesman Nick Clegg also condemned the new revelations, saying: "There is now utter confusion in John Reid's ministerial team."
He said the excuse by the Home Office ministers that the letter they received from Acpo did not explicitly refer to the backlog of 27,000 cases not entered into the computer "suggests that they need to be spoon-fed facts rather than taking their own initiative".
He added: "We urgently need to know who knew what, and when. Ministerial excuses are looking increasingly threadbare."
In his statement to MPs yesterday, Mr Reid said that of the 540 most serious offenders in the backlog of cases, 260 had now been uploaded onto the Acpo database, but admitted 280 people remained unidentified.
He blamed poor details provided by European police authorities for this, and said Acpo was dealing with the problem. The Criminal Records Bureau is also checking to see if any of the 25 rapists and 29 paedophiles had been cleared to work with children.
Mr Reid said that since a new European Union law was introduced last May to make the provision of such information compulsory, the system was more efficient. Acpo took control of the backlog of paper files at this stage and has been going through it since.
However, he acknowledged the situation was "extremely serious".
Earlier, during prime minister's questions, Tony Blair defended the home secretary, saying the 27,000 cases dated back from before his time and were the result of flawed European information-sharing policies - not Home Office incompetence.
"In so far as insufficient details have been sent to us, that is not, with respect, the fault of the Home Office," he told MPs.
He added: "There was of course all the way through an attempt to improve this system but it was only when Europe took the decision to make compulsory what was previously voluntarily that we were able to deal with the backlog."