The number of prisoners in England and Wales has reached a new high for the fourth week running and outstripped prison capacity for the first time.
The government has announced an additional 9,500 prison places
The total stood at 81,533 on Friday, with 333 of those held in police or court cells.
The total number of prison places available is 81,517, with a further 400 cells available in police stations.
The government said contingencies were in place to cope with "short-term population pressures".
The new record number of prisoners compares with the figure of 81,345 last week.
Shadow justice secretary Nick Herbert said: "[Justice Secretary] Jack Straw must now urgently come to the House of Commons to explain how the government got into this appalling position and what plans he has to provide emergency cell capacity.
"Mr Straw must also rule out any extension of early release, which fails prisoners, insults victims and puts the public at risk."
A Ministry of Justice spokesman said the government would "always make sure there are prison places for those serious and dangerous offenders who ought to be in prison".
He added: "We have announced an additional 9,500 prison places and we continue to investigate options for providing further increase in capacity."
Juliet Lyon, director of the Prison Reform Trust, said "pitched battles between government and opposition, union and management" were likely to make things worse.
"The fact is that if we reserved prison for serious and violent offenders only, there would be enough spaces for those who need to be behind bars and staff could do their job effectively," she said.
Meanwhile, prison officers have agreed not to strike at least until the next stage of a legal row with the government is over.
Thousands of members of the Prison Officers' Association (POA) took part in the first national 24-hour strike by prison officers in August .
At London's High Court on Friday, the POA agreed with a Ministry of Justice application calling for no more strike action until a further hearing is complete.
That hearing will revisit an earlier decision declaring the strike illegal.
The POA gave notice in May this year that it no longer wished to be bound by its voluntary agreement not to strike, reached in 2005.
The government says the union had not given the agreed 12 months' warning when it took industrial action in August.
The POA strike followed a government decision to spread the officers' independently assessed pay award over two stages rather than one.
The union says this effectively means a rise of 1.9%, rather than the 2.5% recommended by the Prison Service Pay Review Body.