The West Midlands Police and Crime Commissioner has called on the Prime Minister to outlaw "off-rolling", which sees pupils disappear from school registers even though they have never been formally excluded.

It follows warnings that some schools are getting rid of pupils simply because they are expected to achieve poor results.

And police chiefs say excluded children "are at much greater risk of becoming either perpetrators or victims of serious youth violence."

David Jamieson, the West Midlands Police And Crime Commissioner (PCC), has written to Theresa May jointly with London Mayor Sadiq Khan and the PCCs for South Yorkshire, Humberside, Northumbria, West Yorkshire, Leicestershire and South Wales.

Figures show the number of exclusions is increasing, with the number of young people permanently excluded having risen by 56 per cent across England between 2013/2014 and 2016/2017.

Over the same period, the number of permanent exclusions has increased in the West Midlands from 750 pupils to 1,215, an increase of 62 per cent.

In Scotland, a reduction in school exclusions was seen as a factor in a dramatic reduction in violence over the course of a decade.

Research by HM Inspectorate of Prisons has revealed that nine out of 10 children in custody have been excluded from school.

Chief Constable Dave Thompson and the Police and Crime Commissioner David Jamieson in Birmingham

In the letter, the Mayor and the PCCs say: “There is growing evidence to show that our most vulnerable children are more likely to be excluded or off-rolled from school and additionally that excluded children are at much greater risk of becoming either perpetrators or victims of serious youth violence."

They 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.

“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.

“There is significant variation by school 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.”

Ofsted, the official school inspection service, has said schools may be "off-rolling" pupils as a response to a measurement called Progress 8, in which schools are assessed on the progress pupils make between the end of primary school and the end of secondary school.

Every pupil in Year 11 (which usually means aged 15 or 16), and on the school roll on January 1, is included. So if a pupil isn't doing well, a school might be able to improve its score simply by ensuring the child has gone before they start year 11.

Ofsted’s most recent annual report found that in England around 19,000 pupils - one in 25 - did not progress from Year 10 in January 2016 to Year 11 in January 2017.

In the West Midlands it was around 2,200 pupils.

Lorna Fitzjohn, Ofsted West Midlands Director, said: “In the coming year, our inspectors will be asking questions about off-rolling and exclusions when they go into schools.”

MPs are also concerned. The House of Commons Education Committee says there has been an alarming increase in "hidden" exclusions.

These take place when a child is not officially excluded, but leaves the school anyway. For example, a school might tell a parent that their son or daughter is at risk of being expelled, and it would be better if they voluntarily took the child out of school before that happens.

A YouGov survey commissioned by Ofsted found one in ten teachers said off-rolling had happened at their school.

The letter to Theresa May comes after three teenagers died in knife attacks in two weeks in Birmingham.

Hazrat Umar, 17, was killed in Bordesley Green on Monday; Abdullah Muhammad, 16, died in Small Heath last week; and seven days earlier Sidali Mohamed, 16, was stabbed outside a college in Highgate.

The attacks were among 269 knife crimes recorded so far this year in the West Midlands.