Record numbers of prisoners are being held in prisons and police cells in England and Wales, the BBC has learned.
There are now record numbers of inmates.
Official figures are not due to be released until Friday but the BBC understands there are now 80,199 inmates, including 351 in police cells.
Space in police cells has nearly run out, forcing the Home Office to use cells in courthouses.
Paul Cavadino, chief executive of crime reduction charity Nacro, said prison overcrowding increased reoffending.
The number of inmates has risen from just under 80,000 in February.
Police cells are being used as part of Operation Safeguard, which was implemented to try to ease overcrowding.
But there are only 49 places left, so the National Offender Management Service is planning to use cells at two courts in the South East of England to cope with demand.
Both measures are part of a plan to provide an extra 8,000 cells across England and Wales over five years.
Mr Cavadino said: "The crisis of prison numbers is seriously damaging public safety by pushing up reoffending rates.
"The government should take urgent action to remove the option of imprisonment for low level offenders.
"The law should also require sentences to take account of the capacity of the prison system."
Prison governors are already sending inmates with only two weeks to serve to some of the spare beds that are available in open prisons.
However, the limits of this scheme are understood to have been reached because of the need to ensure inmates are suitable for a less secure environment.
Home Secretary John Reid last month announced plans to build two new prisons - one in Merseyside and one in London.