Charles Clarke, the Home Secretary, admitted tonight that at least five of the foreign prisoners mistakenly released have since been convicted again of serious crimes involving drugs or violence.

A further two prisoners are being investigated for alleged sexual assaults including one case of rape.

Faced with an increasingly loud clamour for his resignation, Mr Clarke also announced that dozens of former prisoners would be rounded up in the coming hours and over the weekend and held for deportation.

In a written statement to Parliament delivered in the form of a letter to the Speaker of the Commons, Mr Clarke reported on progress tracking down the 79 most serious offenders from 1,023 foreign prisoners releases since 1999 without having been properly considered for deportation.

He said that 13 of the 79 had served sentences for murder, manslaughter, rape and child sex offences - the kind of crimes that would entail automatic deportation. Mr Clarke said that deportation proceedings had started against 63 of the former prisoners.

"So far, six of these offenders have been detained pending deportation or removal and further arrests are expected later today and over the weekend," Mr Clarke wrote.

A thorough search of police, prison and probation records had revealed five cases where individuals had been reconvicted for offences "relating to drugs, violent disorder, grievous and actual bodily harm". The drugs offence is for possession of Class A drugs.

The Home Office spoke of "police activity" in two other cases, one relating to serious sexual assault and another an allegation of rape.

Police are still considering the cases. In the case of the offender who was convicted of actual bodily harm there was also an allegation of rape. That was not proven but it remains on file.

It is understood that one of the two "police activity" cases was in relation to someone released after ministers became aware of the problem.

Mr Clarke’s position was regarded as still precarious tonight although the Prime Minister was said to be determined to keep him. His aides said that he was staying in office because he had accepted responsibility both for the mistakes that had been made, and for putting them right.

It is not clear which crimes may have been committed by more than 800 prisoners whom the police have not yet had time to check since being alerted by the Home Office earlier this week - although Mr Clarke said that he will report on those offenders next week.

The initial trawl involved the 79 most serious cases but all 1,023 would be investigated. Mr Clarke would report to the Commons on Tuesday after the May Day Bank holiday and at regular intervals thereafter.

The Home Secretary has so far rebuffed Conservative and Liberal Democrat demands that he resign over the fiasco, especially given the fact that almost a third of those released were freed after the Government had already been informed about the problem.

David Davis, the Shadow Home Secretary, said that the statement had confirmed him in his belief that Mr Clarke should go because those released had committed "very serious crimes, none of which would have happened had the Goverment done its job - had Mr Clarke done his job - of protecting the public".

So far, Mr Clarke has had the full support of Tony Blair, who has twice refused his resignation offers and says that he should accept responsiblity for the problem by staying in his job to sort out the mess.

But two opinion polls published today showed that almost two-thirds of voters think that Mr Clarke should resign over the fiasco. According to a Populus poll for the BBC Daily Politics show, 63 per cent think that Mr Clarke should go, while just 32 per cent want him to stay.

The Home Office admitted earlier this week that it lost track of 916 of the 1,023 released prisoners, including three murderers, nine rapists and five paedophiles, and called in urgent police support to trace them.

The Times reported this morning that the number of foreign criminals freed from prison without being properly considered for deportation was much higher than Mr Clarke has admitted.

It said that hundreds of foreign inmates are believed to have been released on licence in recent years with their papers marked "deportation pending" - and have then disappeared without trace to avoid being forcibly removed from the UK.

BBC Two's Newsnight reported yesterday that out of a list of 80 criminals sent to the Association of Chief Police Officers by the Home Office, 20 did not show up on its computer records, including one murderer and three sex offenders. Mr Clarke's statement said that the 79 most serious offenders - presumably the same list - had all been on the police national computer.

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