Thousands of children are sexually abused at school every year, an investigation has revealed.

More than 5,500 sex offences in schools across the UK were reported to the police in the last three years, including almost 4,000 indecent assaults and more than 600 rapes.

More than 1,500 victims were under 13 and most were primary and secondary school pupils rather than college students or teachers, an investigation by Radio 5 Live has found.

One in five of the attacks were carried out by other children and in some cases the victim and abuser were both as young as five.

Jon Brown, head of sexual abuse programmes at the NSPCC, said: "These figures are very disturbing, especially as many victims are so young and the reported offences took place on school premises.

"Sadly, we are not surprised as previous NSPCC research has illustrated the scale of abuse committed by young people.

Jon Brown, of child abuse charity NSPCC, has blamed hardcore pornography for the scale of attacks (NSPCC)

"We know that for some older children accessing hardcore pornography is warping their view of what is acceptable behaviour.

"And the very young, those of primary school age or even younger, may be copying sexual activity they have witnessed."

Chief Constable Simon Bailey, head of child protection at the National Police Chiefs Council, warned the problem may be even worse than the figures suggest.

He said: "I believe these figures are the tip of the iceberg.

"It is good news that more victims have the confidence to come forward and report abuse, although - while I cannot prove this - I believe more child abuse is taking place.

"That includes children being raped on school premises."

One victim, who asked not to be identified, said he was sexually assaulted in a classroom by three of his friends when he was 15.

He said: "My abusers were the most popular boys in the school, they played on all the sports teams.

"The principal at the time tried to put it down to 'rugby locker-room banter' and didn't seem surprised at all."

A total of 60 children in England were expelled for sexual misconduct in schools in 2013/14, according to the Department of Education.

There were no exclusions in Wales and Scotland. Figures for Northern Ireland were not available.

Anne Longfield, Children's Commissioner for England, said schools and other adults working with children must be more alert to abuse taking place.

She said: "I want personal social health economic education with a relationships and sex component to be part of the national curriculum.

"Every child needs to understand what is inappropriate or illegal behaviour."

A Department for Education spokesman said statutory guidance is "crystal clear" that anyone with concerns about a pupil's welfare should "swiftly" refer it to local authorities or the police.

The information was provided by UK police forces in response to a BBC 5 Live freedom of information request.