The influx of child prisoners accused of involvement in last month's looting and rioting has contributed to an 8% increase in the juvenile prison population in England and Wales.

That calculation is based on Youth Justice Board (YJB) figures which show 170 riot offenders aged under 18 are now in custody, adding to the 2,075 child prisoners recorded in June, the latest statistics available. A Ministry of Justice report out on Thursday suggests a lower tally; it says there are 125 juveniles behind bars for riot offences, with 21 sentenced and 104 on remand.

The Guardian has learned that two-fifths of children in custody have had no previous connection with youth offending teams – a marker of criminal behaviour which resulted in a court order.

The YJB, which has responsibility for the administration of youth justice in England and Wales, says that half of under-18s brought in front of the courts on charges of rioting and looting last month were completely unknown to the criminal justice system and only 10-15% of juvenile rioters had any sort of gang affiliation.

The statistics on minors, who comprise 20% of all those convicted of riot offences, undermine claims from justice minister Kenneth Clarke that the riots were caused by a hardcore criminal underclass.

Campaigners have warned the sudden rise in the number of children in jail was a possible breach of the UK's commitment to protect children's rights. Article 37 in the UN convention states that custody should only be used "as a last resort".

Andrew Neilson, assistant director at the Howard League for Penal Reform, said: "We were very concerned that there was a rush to custody. Normally young people would be recognised as vulnerable and different and that would be reflected... and that wasn't happening. It's been a one-size-fits-all approach.

"Pushing someone from no contact at all to a youth offending institute, which is the deep end is a very risky thing [and]... this is one of the most disproportionate impacts of the riot sentencing"

"The overuse of remand and some of the sentencing certainly puts in doubt the government's claims that child custody is used as a last resort."

John Drew, the chief executive of the YJB, said there was a crisis in young offenders' institutions (YOIs) because of the sudden spike in the prison population. Offenders, especially in London, have had to share cells or have been moved to prisons hundreds of miles away from home.

He said one of the YJB's biggest concerns was the threat of violence against naive entrants to the prison system. Therefore, all those on riot sentences have been categorised as vulnerable, a measure normally reserved for a small minority.

"We've suddenly gained 80 or so youngsters who have not been anywhere near the criminal justice system before," Drew said. "They go into custody and there all sorts of codes and ways of getting through a sentence that is alien to them. So the establishments have treated them all as vulnerable and have responded to try to help them understand what is expected of them from prison officers and what may or may not be acceptable in terms of how the other inmates will expect from them. But inevitably it's a new mixture... It's a real challenge to our staff."

Drew said he knew of a "couple of instances" of violence involving new entrants. An incident in Cookham Wood young offenders institute left two children in hospital.

A report sent to the Guardian by the Howard League for Penal Reform, suggested that turf wars had broken out in one YOI.

The summary of a phone conversation with a young offender reported by a solicitor working with the Howard League read: "Prisoners who were involved in the riots have been advised to walk around in pairs and to not be out in the landing on their own. Other prisoners are saying that the rioters have 'destroyed their turf' and beatings have been going on all day. It was difficult to hear on the telephone as screaming and shouting could be heard in the background."

In a major speech a week after riots, David Cameron announced he would declare "a concerted, all-out war on gangs and gang culture" which he believed were the root cause of the looting and destruction of property across English cities.

In interview at the YJB's Pimlico offices, Drew clashed with home secretary Theresa May, warning that any relaxation in the strict anonymity rules governing the identification of young offenders would only allow young offenders to revel in their criminal status.

A week after the riots, May said the Crown Prosecution Service"should be asking for the anonymity of juveniles who are found guilty of criminal activity to be lifted".

Drew believed that May was "responding to a public appetite" but that it was right that the lifting of anonymity remained "very rare".

"I started my working life with young offenders and the kids were crying out to be told they were young villains because they got status from that. Take that status away from them, try and treat them in a different way is probably the single most effective thing to do," he said.

Drew admitted that the situation with overcrowding was not ideal: "It's a real dilemma and it is essentially about cost. We now have 2,150 under 18s in custody. In a perfect world, you would have a small YOI in every town."

"When you get an incident like this with a particular focus in London, it's inevitable you're going to get congestion. So, in the immediate aftermath, we had to move some children north. We tended to move kids who were on a sentence in order to make room for those remanded. But there's no secret in that, we did have to move kids around. It is simply about supply and demand and our capacity to afford the ideal."