Children as young as 11 were held in police cells last year because officers thought they were mentally ill, an investigation has revealed.
Two of the 347 under-18s, who had not necessarily committed a crime, were detained for more than 24 hours, the report on Radio 4's The World At The Weekend revealed.
The law allows police to take people they think are mentally ill and in "need of care or control" to a safe place for a health assessment.
Two forces detained one child each for longer than 24 hours, figures obtained by the programme through Freedom of Information requests showed.
The legislation allows detention for up to 72 hours.
Out of the 42 forces in England and Wales that provided data, 35 held children under the age of 18 in custody under the Mental Health Act last year.
Usually a place of safety for assessment would be a hospital or care home, but police stations are also permitted under the Act.
Health Minister Norman Lamb said the Department of Health is working with the Home Office and police to provide better local services to ensure a "health-based place of safety" is available as soon as possible.
"All services should be working together to minimise the chance that children and young people with a mental illness end up in a police cell," Mr Lamb said.
"Using police cells in this way should happen as little as possible and only where there are circumstances where it is unavoidable or where the person's behaviour poses an unmanageably high risk.
"The Department of Health is working with the Home Office and the Association of Chief Police Officers to develop better local protocols between police and mental health services so that people who are found in immediate need of care and control can get the most appropriate service and, where needed, a health-based place of safety as soon as possible.
"We have invested a total of £54 million over the four years to 2014-15 to deliver better mental health care to children and young people when they experience mental health issues."
The Association of Chief Police Officers lead on mental health, Chief Constable Simon Cole, said: "It's quite clear that, in some places, police officers who are finding vulnerable young people at a point of crisis have no realistic option other than to take them to cells rather than a health-based location.
"I don't think that anyone looking at that situation would think that that's the best answer."
Mr Lamb added that holding children in police cells should be "the absolute exception, not the rule".
He told The World This Weekend: "They should be getting access to appropriate treatment straight away. To place youngsters in police cells is completely unacceptable in all but the most exceptional circumstances."
Asked if he would consider banning children aged under 14 from being held in cells, Mr Lamb said: "I want to look at that and I want to work with Young Minds (a children's mental health charity) and other campaigners to make sure we get this right.
"There may be some limited circumstances where a youngster, perhaps for their own safety, just has to be placed somewhere but this must be the absolute exception, not the rule.
"I suspect that this practice has always happened and the fact that we can now expose what's happened so we can actually bring this to an end is a good thing.
"I personally find this completely unacceptable."