Emergency measures to reduce prison overcrowding will not expose the public to more dangerous criminals, Attorney General Lord Goldsmith has promised.
The prison population is approaching 80,000
Home Secretary John Reid is expected to announce that some inmates will be moved from secure jails to open prisons and police cells.
Lord Goldsmith said public safety would come first, and no prisoner would be moved without a proper risk assessment.
Meanwhile, the Lord Chief Justice has urged more use of community sentences.
The number of people in prison has reached a record 79,843, and in theory there are just 125 more spaces available.
Lord Goldsmith told BBC News: "If there were a significant risk of someone being a danger, they wouldn't be moved to an open prison."
He added that only "a very small proportion of the overall prison population" could be suitably punished without using custody.
According to the Sunday Times, Fiona Radford, governor of Ford open prison in West Sussex, warned in August that if more inmates were transferred from secure prisons, there was a risk of more absconds and drug use.
She reportedly said in a memo to staff that she had informed the Prison Service.
A risk of more drug abuse and absconds had been "accepted as inevitable" by Mr Reid, the newspaper quotes the document as saying.
But Home Office minister Gerry Sutcliffe said: "John Reid and I certainly wouldn't want to put the public at risk.
"Any re-categorisation has got to be risk assessed and people have got to have not committed violent crimes or sexual crimes.
"No violent offenders or sexual offenders should be moved from category C to category D."
The Lord Chief Justice, Lord Phillips of Worth Maltravers, has urged more use of community sentences, saying prison overcrowding made it difficult to rehabilitate inmates.
'Dangerous or inadequate?'
He told the Observer: "The idea that alternatives to custody is being soft is wrong."
The public must be educated to distinguish between the "brutal, dangerous offender and the inadequate who offends to get money for drugs".
Lord Phillips made his comments to the newspaper after going undercover to serve part of a community order doing manual work.
Shadow Home Secretary David Davis said more focus on drug rehabilitation and work-related training was needed in prisons.
Liberal Democrat home affairs spokesman Nick Clegg warned that open prisons must not become "dumping grounds" for dangerous criminals.
Norman Brennan, director of the Victims of Crime Trust, said the government had an "obsession with keeping dangerous and persistent criminals out of prison".