Police officers have been forced to look after 20,000 prisoners in the past year due to the jail overcrowding crisis, it has been claimed.
The Liberal Democrats published figures which suggest £2.6 million a month is being spent on the emergency measures, which see offenders housed in police stations rather than jails.
The party produced the overall total from extrapolations of quarterly figures.
It said Parliamentary answers from Government minister Gerry Sutcliffe showed that between October and December last year police cells were used on 4,617 occasions. Between January and March they were used on 9,380 occasions, which averaged at 140 a night, or 20,437 in the past year, the Lib Dems added.
They also estimated the cost of dealing with prison overcrowding is £2.6 million a month, or £87,000 a night.
Lib Dem leader Sir Menzies Campbell said: "The job of our police forces is to catch criminals, not to look after them by becoming full-time jailers.
"Police officers are already overburdened by bureaucracy and targets. This is one more thing that prevents them doing what they are paid for. Using police cells is a 'quick fix' solution which will not make the prison crisis go away."
He added: "The UK has the worst reoffending rates in Europe, with prison acting too often as a revolving door for criminals.
"We need to make prison work by making education and work compulsory and ensuring that non-violent criminals work in their local community to make amends for their crimes."
Sir Menzies is due to address the Police Federation annual conference in Blackpool.
