The Home Secretary John Reid has clashed with the Conservatives over reports the Home Office ignored files on serious offences by Britons abroad.
Of 540 serious criminals - including rapists and murderers - 280 are still not on the Police National Computer. Mr Reid has promised a full inquiry.
There is concern offenders could have been cleared to work with children.
Tory leader David Cameron said Mr Reid "won't be able to run away" from responsibility if this has happened.
Mr Cameron was speaking during a heated Prime Minister's Questions with Tony Blair, before Mr Reid made his statement.
On Tuesday, the Association of Chief Police Officers (Acpo) said details of 27,529 cases, including 25 Britons convicted of rape, were left in files at the Home Office.
The association said they should have been entered on the Police National Computer (PNC).
In his statement to the Commons, Mr Reid said that Acpo had entered details of 260 of the 540 serious criminals.
"The remaining 280 cannot be entered on the computer and are the subject of further inquiries to the notifying country to get more details to try to establish the identity of the offender," he added.
Shadow home secretary David Davis attacked the department's recent record as a "catalogue of blunders".
"The last three years have been the worst three years in the Home Office's 200-year history," he said.
Meanwhile, a row has broken out over a letter that Acpo sent to Home Office minister Tony McNulty last October, which said the association was having difficulties tracking down the criminals.
ACPO said the letter to Mr McNulty, the contents of which it will not release, did not mention the backlog issue.
And the Home Office said ministers were not aware of the backlog until Tuesday.
Mr Davis said the position of two Home Office ministers would be "untenable" if emerged that they knew about the missing information surrounding British offenders committing crimes abroad.
"We need to see the full copy of this letter. ACPO should now publish it including all replies," he said.
"It is also the case that any inquiry cannot be carried out by a civil servant. It must be carried out by a judge."
An inquiry into the Home Office's handling of the details was expected to be completed within six weeks, Mr Reid said.
The Criminal Records Bureau (CRB) would be checking if any disclosures to employers about the most serious offenders needed to be examined again, he added.
He said he expected to be told conclusions "in a matter of days".
Zoe Hilton, from children's charity the NSPCC, said they were "very worried" about the revelations.
"Parents need reassurance that actually these people aren't working with children and they are being urgently traced," she said.
Earlier, Home Office minister Joan Ryan said details of the offences were still being registered by police.
She also told BBC News "some answers" were expected on Wednesday as to whether dangerous offenders were working with children.
Responsibility for updating the records was transferred from the Home Office to Acpo last year.
The cases involved included:
25 rapes3 attempted rapes29 paedophiles17 other sex offenders5 murders9 attempted murders13 manslaughter convictions29 robberies
The details emerged during evidence given by Acpo spokesman Paul Kernaghan to the Commons all-party home affairs select committee on Tuesday.
He said information on the cases had been supplied to the UK, but had been "sitting in box files" until his organisation took responsibility for them.
And it has emerged that Acpo asked for extra funds to help clear the backlog in October.
The request went to a senior Home Office official, but was declined.