Theresa May is being warned that a "broken" system of support for troubled and excluded youngsters lies at the heart of a rise in knife crime.

Police chiefs and London's mayor have written to the PM saying how pupils, both formally and informally excluded, are being "sucked into criminality".

The letter says cuts to school funds and youth services mean "interventions" for needy youngsters are not happening.

The government said permanent exclusions should be a "last resort".

A spokesperson added that pupil referral units, where children excluded from mainstream school are taught, had a legal duty to safeguard children from dangers such as exploitation, abuse and gang activity.

The letter also called for unofficial exclusions, known as off-rolling, to be outlawed.

It comes a day after high-level talks between the home secretary and police chiefs and follows a spate of fatal teenage stabbings.

On Wednesday, Mrs May said the deaths of young people were "appalling" and announced a summit on knife crime.

The letter to Mrs May - signed by Mayor of London Sadiq Khan, six police and crime commissioners in England, and one in Wales - said tackling the issue was their top priority, but added there were limits to what they could do.

"There is growing evidence to show that our vulnerable children are more likely to be excluded or off-rolled from school," it said.

"And additionally that excluded children are at much greater risk of becoming either perpetrators or victims of serious youth violence."

It calls for the system of off-rolling, which sees pupils disappear from school registers without having been formally excluded, to be banned.

The letter said: "We firmly believe it is unacceptable that young people can be ejected from the formal education process in this way when we know how vulnerable they become to being sucked into criminality as a result.

"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 highlighted that in two of the country's knife crime hot spots, London and the West Midlands, permanent exclusions rose by 62% and 40% respectively since 2013-14.

It added: "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."

It said it was time to act urgently by giving the responsibilities over all school exclusions back to local authorities.

Rona Epstein, research fellow at Coventry Law School, said in a recent study that parents of children with special educational needs often found there was "little or no support available" and faced "extremely long waits for diagnoses which could help get that support".

"Some parents had even been asked to keep their children at home because schools said they could not meet their needs," she said.

Last November, Unison highlighted how youth centres have been affected by funding cuts.

A Freedom of Information request by the union revealed how at least 760 youth centres had closed in England since 2012 - leading to the loss of 4,500 youth work jobs.

An Ofsted spokesman said it had seen no convincing evidence that exclusions, in and of themselves, lead to knife crime or gang violence.

It was, however, 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," he said.

A government spokesperson said excluded children should still be able to engage in high-quality education and £4m had been committed to improve outcome for children outside mainstream schools, including in pupil referral units.

"Since 2017, the UK government has given every local authority in England more money for every pupil in every school," the spokesperson added.