A police force has been told to take "immediate action" on how it deals with children in custody after one was held for eight hours without review, a report found.
South Yorkshire Police was not "consistently" meeting the requirements of codes of practice for detention under the Police and Criminal Evidence Act and the Children and Young Persons Act 1933, inspectors said.
Around 100 cells in Barnsley, Doncaster and Sheffield custody centres - which held 22,229 people between June 2018 and May this year - were examined.
While the force had made some improvements, there were "several causes of concern and areas that required improvement", the inspection in June found.
A joint report by Her Majesty's Inspectorate of Constabulary and Fire & Rescue Services (HMICFRS) and HM Inspectorate of Prisons made a series of recommendations for improvement, including over concerns raised about how the force was in breach of rules surrounding children in custody.
The report said: "The force had few arrangements in place to provide children with specific support.
"They were not prioritised during the booking-in process, which meant they could be held with adult detainees in the holding areas, especially when it was busy.
"No easy-to-read rights and entitlements documents were available to help them understand their rights, although a photocopied guide to custody for children was sometimes handed to them.
"Although it was the force's policy to review the detention of children every four hours, the records we looked at did not demonstrate that this was happening consistently.
"In one case, a child left custody after eight hours without having had a review of detention.
"In addition, girls were not consistently in the care of a woman as legally required by the Children and Young Persons Act 1933."
The force "must take immediate action to ensure that all custody procedures comply with legislation and guidance, and that officers implement them consistently," inspectors said.
Officers also should make sure the use of force was "safe and proportionate to the risk or threat posed" and detainees should only be strip-searched when necessary while making sure their dignity is respected.
Chief Inspector of Prisons Peter Clarke and Inspector of Constabulary Wendy Williams said in a joint statement: "While we found a number of positive features, there were several causes of concern and areas that required improvement.
"However, the force had made some progress since our last inspection in 2014 and was open to external scrutiny, which meant we were confident that it would take action to improve."