Police chiefs have warned Theresa May a “broken” school exclusion system is linked to a surge in knife crime.

In a letter to the prime minister, co-signed by London mayor Sadiq Khan, seven police and crime commissioners in England and Wales call for an end to unofficial “off-rolling exclusions”.

“Clearly, the way the education system deals with excluded young people is broken,” the letter says.

“It cannot be right that so many of those who have committed offences have been excluded from school or were outside of mainstream education.

“That is why the time has come to act urgently. In the first instance, local authorities need powers and responsibilities over all school exclusions.

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“Time and again we are hearing how the fragmentation of the education system, and the breaking of the link between schools and local authorities, has led to a lack of accountability, co-ordination and action.”

The letter follows a sharp rise in exclusions in London and the West Midlands – two areas worst-hit by knife violence.

Figures show permanent exclusions in England increased by 56 per cent between 2013-14 and 2016-17.

There was a 40 per cent rise in London and 62 per cent rise in the West Midlands during that period, according to Department for Education data.

The letter adds: “There is significant variation by schools as to what will result in exclusion, with many excluded pupils moving between local authority areas and also out of their cities.

“The practice of off-rolling must be outlawed.”

The government is also urged to increase funding for schools to improve early intervention for children at risk of exclusion.

“Our schools are facing significant funding pressures and many interventions for our most vulnerable children are being cut," the letter says.

"This cannot be right and schools must have the necessary resources to deliver good interventions and support to those at risk of exclusion.

“We are investing in our policing as much as the Government will allow us to do, plus further investing in early intervention projects across our regions.

“Yet so many of the causes of violent crime are out of our control, but in the hands of the government.

“That is why it is high time the government matched our ambitions and showed clear leadership on this issue.”

West Midlands Police and Crime Commissioner David Jamieson said last week knife crime was a “national emergency”, with excluded children the most likely to be involved.

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He said: “We must do something about that exclusion of children because those children are on almost an immediate path into crime and into violence.”

Commissioners for the West Midlands, South Yorkshire, Humberside, Northumbria, West Yorkshire, Leicestershire and South Wales signed the letter.