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Vulnerable children were held overnight in police cells an alarming 100 times every week last year, figures showed today.

Scotland Yard recorded more than 3,000 cases of under-18s behind bars for at least four hours between midnight and 8am over a six-month period.

The detentions occurred even though the law states children should be transferred to local authority accommodation wherever possible.

Labour claimed there was a “serious problem” and that council budget cuts and housing shortages meant children were being put at risk in custody.

Figures provided by the Mayor revealed that under-18s were kept overnight in custody 3,005 times between November 2014 and May 2015, the latest period for which data is available.

There were a further 483 incidents of youngsters being held over the weekend.

By law the police cannot release a child from custody without an appropriate adult being present. If nobody is there to collect them, councils have a legal duty to provide temporary accommodation to avoid the youngster being left in a cell overnight.

Town halls claim that some vulnerable children are left languishing behind bars because of the “acute shortage” of secure temporary accommodation, exacerbated by budget cuts.

They called on the Government for more resources to help children in custody.

In a written answer, Boris Johnson said: “The Commissioner is committed to ensuring that all detainees, including young people, are protected and treated appropriately while in police custody.

“It is vital that appropriate safeguards are in place for all under-18s who come into contact with the police and that they are not held overnight in a police cell unless it is impractical to transfer them to local authority accommodation.”

But London Assembly member Andrew Dismore, who obtained the figures, said: “The law is very clear that nobody under 18 should be left in cells overnight.

"The fact that vulnerable under-18s were held overnight in London cells over 3,000 times in six months alone shows there is a serious problem.

“The police tell me that a combination of budget cuts and housing shortages are having a devastating impact on councils’ ability to place young people and prevent them spending the night in a police cell.

“It cannot be right that young people, many arrested on relatively minor offences, end up forced to spend the night in a cell just because the Government won’t give councils the resources they need to provide safe alternative accommodation.”

Councillor Lib Peck, Labour’s crime and public protection spokesman on London Councils, said: “Boroughs take very seriously their duty to protect vulnerable children. The policy is to ensure that young persons are not held in police stations when other secure accommodation is available.

“Whenever a request is received from the police every effort is made to find a local secure place but there is an acute shortage of secure places.

“Local government is facing cuts of up to 60 per cent in central government funding and a huge increase in demand for temporary accommodation as homelessness increases.

“Government must act on the housing crisis and provide sufficient resources to significantly increase the number of secure accommodation units.”