The prison overcrowding crisis should be tackled by sending fewer criminals to jail, according to the Chief Constable of West Midlands Police.
Paul Scott-Lee said many criminals languishing in cells might need treatment for mental illnesses rather than locking up.
He was speaking as an official Home Office report named West Midlands Police as one of the top forces in the country.
Official inspectors said it was performing more impressively than other major urban forces such as Merseyside or Greater Manchester police.
They highlighted the "excellent" work of West Midland officers in investigating crime and bringing offenders to justice. The same report gave Staffordshire the highest score of any force in the country.
John Reid, the Home Secretary, has announced a range of measures to tackle the problem of overflowing jails. Plans include converting a barge into a prison ship for foreign prisoners and asking police to use their cells for overflow inmates.
A building programme will also create 1,100 new prison places by the end of next year. And yesterday the chairman of the Youth Justice Board warned the service was in "meltdown" with only a handful of beds now available.
Mr Scott-Lee said: "There is a national problem with all places of custody, because we have been locking up more people. We have been arresting them and the courts have been giving more custodial sentences.
"We are probably at saturation point with the numbers that currently exist.
"There is a prison building programme and I have seen the news reports of the possible use of a prison barge.
"But you also have to look very hard at who you are locking up. A large proportion have mental illnesses. We must ask ourselves whether or not prison is the best place for them."
Mr Scott-Lee believes people who commit violent crime or are "a menace" to society should be jailed.
But he added: "The question has to be raised as to whether or not, perhaps in addition to providing more prison places, we would be better off improving mental health services."
According to the Howard League for Penal Reform 80 per cent of prisoners have a history of mental health problems or learning difficulties, while 20 per cent have a serious diagnosable problem such as personality disorder.
The Chief Constable was backed by the Howard League's Director, Frances Crook. She said: "The answer is to
provide a range of services in the community which provide long term care."
Mr Scott-Lee also warned that the strong performances of forces such as West Midlands Police, West Mercia Police and Staffordshire Police did not remove the need for major reforms.
Earlier this year the Government backed down over plans to merge the region’s four forces, including Warwickshire police. It followed a campaign by West Mercia against the proposal, but the other three forces had all supported the idea.
Mr Scott-Lee said: "We are making progress now, but we could make even more if we worked together.
"I don't think this debate has gone away, it has just been parked for a little while."
The Home Office report named Staffordshire as the best force in the country, while West Mercia Police was the best in its category, which includes rural forces such as Cambridgeshire and Suffolk.
Warwickshire, however, received one of the lowest scores in the country, placing it 37th out of 43 forces in England and Wales. Its record for investigating crime was "poor", the report said.
Warwickshire Chief Constable Keith Bristow last night stressed that he still supported the merger as the way to tackle the problems faced by smaller forces like his.
"We had hoped the move towards larger and more strategic forces would have enabled us to improve our service further in areas such as major incidents and serious and organised crime," he said.
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