Police chiefs have written to Prime Minister Theresa May, warning that a "broken" school exclusion system is linked to a surge in knife crime.

London Mayor Sadiq Khan co-signed the letter, which calls for an end to unofficial "off-rolling exclusions".

Schools have already been warned by a government minister that it is illegal to unofficially exclude children and that the practice must end. 

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Today's letter states: "Clearly, the way the education system deals with excluded young people is broken.

"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.

"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, coordination and action."

The letter, signed by six Police and Crime Commissioners in England and one in Wales, 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 a 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. This cannot be right and schools must have the necessary resources to deliver good interventions and support to those at risk of exclusion," the letter says.

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

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."

Last week, the West Midlands police and crime commissioner suggested exclusions were a factor behind the violence, and London’s deputy mayor for policing and crime has also suggested there is a connection.

But the idea of a link between school exclusions and knife crime has been challenged with some saying a link is likely to be about correlation rather than causation.

Ministry of Justice analysis published last year concluded: "The low volumes of knife possession offences following exclusions mean any such association could not be a significant driver of youth knife possession offending overall."

Earlier this week a senior Ofsted official defended a report that rated a school as "good", despite finding that it had off-rolled pupils for its own benefit.

The inspectorate has been criticised on social media for its report on Discovery Academy in Stoke, which was found to be off-rolling pupils in Year 11.

Sarah Robinson, the chief executive of Alpha Academies Trust, which runs the school, said the timing of when pupils were removed from the school roll was based on a city-wide agreement which aimed to give pupils the best chance to be able to return to mainstream education. 

She told Tes this was not an attempt to remove pupils who were about to fail but Ofsted had decided that it met the inspectorate's definition of off-rolling.

An Ofsted spokesperson said: "We have seen no convincing evidence that exclusions in and of themselves lead to knife crime or gang violence. It is just as likely that exclusions are caused by the same underlying factors as violent crime, and therefore affect many of the same young people. Exclusions made in accordance with statutory guidance and leading to placement in a good-quality, registered alternative provider should help keep all children safe.”

A government spokesperson said: “Permanently excluding a child from school should only ever be a last resort, and pupil referral units have a legal duty to safeguard children, protecting them from exploitation and abuse including gang activity.

“It is still vital that young people who are excluded from school are able to engage with high-quality teaching and education. That’s why, as part of our commitment to reforming alternative provision, we have launched a £4 million fund which is delivering projects to improve outcomes for children in alternative provision, including pupil referral units."