Prisoner numbers in England and Wales have hit a record high for the second week in a row, says the Prison Service.
The government plans to create 8,000 new prison places
Numbers reached 80,812 - up 154 on last week which leaves just 320 spare places including those in police cells under "Operation Safeguard".
Responsibility for prisons has passed to the new Ministry of Justice, which has announced plans to reduce numbers.
Minister David Hanson told the BBC: "People are inside more because we're protecting the public."
Measures under consideration include a review of sentences, a cap on sentences for prisoners who breach their licence, and restrictions on the types of crimes which are punishable by suspended sentences.
Mr Hanson told the BBC the ministry's prime duty was to protect the public, and more crimes were being solved - which was one of the reasons prison numbers were going up.
But he added: "I accept we need to do stronger planning, we need to bring forward the prison building population and we need to look imaginatively - I hope with the support and confidence of the public - at how we can undertake alternative sentences."
Prime Minister Tony Blair said earlier in the year that prisons were at "bursting point" and chief inspector of prisons, Anne Owers, has said the jail system was in "serious crisis".
The Home Office says it has created 20,000 more prison places since 1997 and plans to create another 8,000 over the next five years - including building two new prisons in Merseyside and in London.
But Charles Bushell, the head of the Prison Governors' Association, told BBC Radio Five Live the only serious way to reduce reoffending, was to reduce the number of people being sent to prison.
He added: "The general feeling from prison governors is that the figures seem, inexorably, to be going up.
"We spend our whole lives hoping and praying that more accommodation comes on line in time to give us a little bit of temporary relief."
Operation Safeguard came into force in October 2006 when prisoners were placed in police cells to try to ease overcrowding.