A mentally ill man left in a cell for three days by a health trust was held illegally, the police watchdog said.
The Independent Police Complaints Commission said the trust put officers in an extremely difficult position.
The sectioned man, 34, was arrested for possessing an offensive weapon and held in a Manchester police station until a hospital bed could be found for him.
A spokesperson for Manchester Mental Health and Social Care Trust agreed the delay had been "unacceptable".
Police arrested the man on 7 March 2005 and took him to West Didsbury police station, Manchester, where the man was diagnosed as being mentally ill.
Despite being bailed by Greater Manchester Police the next morning, delays in finding accommodation meant it took until the afternoon of 10 March before he was transferred to Wythenshawe Hospital, Manchester.
Naseem Malik, IPCC Commissioner for the North West, said Greater Manchester Police, who had acted professionally, were left with a mentally ill man who they were told was a danger to the public.
"The officers clearly could not release the man, even though they were technically detaining him unlawfully after his bail was granted," he said.
"Every effort was made by the police officers involved to expedite the man's transfer to suitable accommodation but inaction by representatives of Manchester Mental Health and Social Care Trust severely hindered the police."
The trust has expressed regret about the delays in the case, which it said highlighted difficulties in assessing the care needed when a patient is unknown to the local service.
Eventually an intensive care bed was found for the man after unsuccessful attempts to arrange a placement in other NHS Trusts and in private facilities.
A spokesperson said: "We agree with the IPCC that such a delay was unacceptable and we have been working with the police and health commissioners to ensure that any delays are kept to the minimum in future.
"We are developing a protocol with the police to ensure that a senior manager takes overall responsibility until a bed is found."
Mr Malik said the case highlighted a wider debate about whether police cells are appropriate places for people suffering from mental illnesses.
He said police stations could not be used as "an extra arm of the health service".
Greater Manchester Police said its officers had found themselves in an "extremely difficult situation" with the man and decided to keep him in custody to protect the public.
"While we appreciate the difficulties faced by the mental health trust, we do believe that people with mental health problems should be given the appropriate medical treatment as soon as possible, and in the right environment, in order to avoid causing additional distress to themselves and others," a spokesman said.
"The force readily seeks to support any protocols devised which will prevent any incidents of this nature recurring in the future."