Inspectors have deemed the way police cells in Lincolnshire are run as "disappointing" and "continually failing" in their latest report.
Her Majesty's Inspector of the Constabulary and the Chief Inspector of Prisons saw cells in Lincoln, Boston, Skegness, Grantham and Spalding.
The joint report found "too often", those with mental health problems were detained and not sent for NHS care.
A Lincolnshire Police spokesman said changes were in progress.
Chief Inspector of Prisons, Nick Hardwick, said: "We were concerned that about 500 people a year are detained in Lincolnshire who have been sectioned under the Mental health Act and they are brought to Lincolnshire Police cells as a place of safety.
"I think we recognise across the police service that police cells are not a proper place to detain people who are mentally ill and provision should be made for them by the NHS.
"Overall, this is a disappointing inspection. It came at a time when the force was undergoing significant change, which perhaps helped explain why we identified continued failings..."
Assistant Chief Constable Keith Smy said; "While I share [the inspectors'] disappointment about issues we have been aware of for some time and have been progressing as a police service and with other organisations, we cannot dictate the pace at which other people make changes and have to provide a plan B."
Other areas of concern in the report included the "variable" quality of risk assessments and the inconsistent health and safety monitoring.
Mr Hardwick also found areas to praise in his report including the professional conduct of staff in interactions with detainees and good substance misuse services.