Being excluded from school is often the "tipping point" that leads to children picking up knives, a group of MPs and peers has warned.

Youngsters who are taken out of lessons are at risk of becoming involved in violence and being exploited by gangs, according to a report by the All-party Parliamentary Group on knife crime.

It is calling for an overhaul to break the links between school exclusions and knife crime.

More than 17,500 boys aged 14 in England and Wales carry a knife or weapon, the cross-party group said, and a third of those arming themselves have had weapons used against them.

Official figures show that children in England's schools were permanently excluded on 7,900 occasions in 2017/18 - a 70% increase since 2012/13.

There was also a 54% rise in fixed-term exclusions compared to 2012/13, the report adds.

The group, made up of more than 50 MPs and peers, says that mainstream schools need to be more accountable for the children they exclude and action is needed to ensure that excluded pupils get a decent education.

"Too many children are being socially excluded and marked as failures, with tragic consequences," the report says.

"All too often the moment of school exclusion is the tipping point that leads to young people picking up knives.

"It's increasingly clear children outside of mainstream schools are at serious risk of grooming and exploitation by criminal gangs. Professionals talk of the 'PRU (pupil referral unit) to prison pipeline'. We must act now to stop the flow."

The group said it had heard from professionals that criminal gangs are seeking to exploit school exclusions, reporting cases where it was thought gangs had set out to engineer exclusions, for example by giving a youngster a knife to take into school.

"The gangs knew that school exclusion would increase a young person's vulnerability and make it easier for them to be exploited into criminal activity," the report says.

One child told researchers: "Since they kicked me out I've got time on my hands to do more crime, commit more crime ... in Croydon with my friends who have also been kicked out, who are also doing wrong things, who are also selling drugs, who are also carrying knives."

Figures obtained by the PA news agency through recent Freedom of Information requests showed thousands of children have been caught carrying weapons in school, with suspects as young as four-years-old.

Knives, blades and a sword were among the weapons seized by police.

The report says some of the children it spoke to suggested a rise in "zero tolerance" behaviour policies meant that "it seemed that schools increasingly relied on both fixed term or even permanent exclusions to respond to what seemed to be relatively minor behaviour".

Part of the reason for a rise in school exclusions is down to "perverse incentives" created by school accountability and inspection systems, which mean headteachers are under pressure to get good Ofsted reports and boost standards, it suggests.

In addition, some schools are excluding pupils because they struggle to manage their behaviour and to find the resources to support them.

Children who are excluded are legally entitled to full-time education after six days, the report says, but this is often not happening.

Some youngsters are being offered only part-time education and many councils are struggling to find places for excluded youngsters in alternative state-funded settings, such as pupil referral units.

The group sets out a series of recommendations, including a call for a government review of the use of part-time education for excluded pupils.

Sarah Jones, APPG chairwoman and Labour MP for Croydon Central, said: "The number of children being excluded from school and locked out of opportunities is a travesty. Often these children have literally nowhere to go. They are easy pickings for criminal gangs looking to exploit vulnerable children.

"Excluding children must be a last resort. But we hear all too often of schools stretched too thin to provide the wrap-around support struggling children need. Cash-strapped councils can't manage the increasing number of excluded children in need of alternative education."

Javed Khan, Barnado's chief executive said: "We know that exclusion too often leads to a 'poverty of hope' - reducing a child's chance of gaining good qualifications and entering the workplace.

"With PRUs regarded as a recruiting ground for criminal gangs, it's no surprise children taught there are vulnerable to involvement in drugs and violent crime."

An Ofsted spokesman said exclusions are a "vital measure" for headteachers, but that they must be "legal and justified".

"Inspectors will consider whether the school is developing the use of alternative strategies to exclusion and taking account of any safeguarding risks to pupils who may be excluded," he said.

"Inspectors will recognise when schools are doing all that they can to support pupils at risk of exclusion, including through tenacious attempts to engage local support services."

Paul Whiteman, general secretary of the National Association of Head Teachers, said: "The principle of a joined-up approach to young people's safety is sound.

"But schools already work in partnership with the police and local authorities to prevent pupils from coming to harm and to ensure they don't cause harm to others.

"The problem for all these agencies is that they are all under-resourced and over-stretched."

A Government spokeswoman said: "The Government has been clear that it will always back teachers and headteachers in delivering discipline in the classroom.

"The issues surrounding knife crime and poor behaviour in schools are complicated and multi-faceted. Simple causal links between exclusions and crime cannot be drawn.

"We are clear that permanent exclusion should only be used as a last resort and exclusion from school must not mean exclusion from education.

"Furthermore, we must be just as ambitious for young people in alternative provision as we are for those in mainstream schools, and we are taking a range of actions to drive up the quality of those settings."