After Devon and Cornwall Police force complained it was forced to hold a 16-year-old girl with mental health problems in a cell for two days because there was no hospital bed available, the Home Secretary is expected to announce that teenagers who suffer from mental health problems will no longer be held in police cells.

Theresa May is understood to be revealing an overhaul of mental health laws in England and Wales, which will also see police cells used only as a place of safety for adults when their behaviour is so extreme they cannot be managed elsewhere.

The joint Home Office and Department of Health review of sections 135 and 136 of the Mental Health Act will also reportedly recommend reducing the maximum length of detention of someone in mental distress from 72 to 24 hours.

Charities and care providers cautiously welcomed the move but warned beds must be available in order for the reforms to work.

Sarah Brennan, chief executive of charity YoungMinds, said: "It is appalling that children and young people are held in police cells because there is no other place of safety for them.

"However, this legislation alone is not the answer. There must be funds available so that there is an increase in the number of safe appropriate beds that local areas can access easily and quickly.

"If this does not happen then the change to the law will have a knock-on effect that will see children and young people transported hundreds of miles across the country or placed on adult wards which the Mental Health Act already states should not happen.

"Even worse, as the CQC recently reported, commissioners are complaining that, where there are 'health places of safety' young people are very often refused access because they are said to be 'too vulnerable' to be placed there.

"Changing the law is a laudable action but this must be backed up by fully resourced locally based, appropriate provision which fully meets the needs of children and young people undergoing the truly frightening experience of a mental health crisis."

Efi Hershkovitz, chief executive and founder of private care provider for young people Danshell, said: "This is a step in the right direction but the Government must ensure that 'place of safety' beds are available for these vulnerable young people. It is not enough to pay lip service to the problem."

He added: "The private sector can provide a solution to the problem of a massive shortage of beds and indeed providers have beds available in current operating units but due to contracted bed restrictions these cannot be utilised."

The move comes after reports that Devon and Cornwall Police force complained it was forced to hold a 16-year-old girl with mental health problems in a cell for two days because there was no hospital bed available. She was eventually placed in an adult hospital bed.

The number of times police cells have been used to hold mentally ill people has fallen, dropping from 8,667 occasions in 2011-12 to 6,028 in 2013-14.

But they are still being used instead of hospital beds or community places in more than a third of cases.