Hundreds of children arrested by the police are being locked up before trial by “risk-averse” judges, who often view them as gang members, according to a significant new report.

The research, the first of its kind for almost a decade, paints a picture of a criminal justice system that views children as “mini adults”, with the result that many are remanded into custody with potentially traumatic consequences. The “overuse” of pre-trial detention – described in the report by Transform Justice, the national justice charity – is at odds with what happens to most of the children post-trial. Of those remanded, two-thirds do not go on to receive a custodial sentence.

Measures introduced in 2012 by the coalition government were supposed to reduce the number of children on remand, but they now make up a quarter of the child prison population compared with one-fifth when that legislation was introduced. In June, 30% of children in custody were on remand, the highest monthly figure for 10 years. Last year, children were remanded in custody 1,269 times.

The new report also highlights the disproportionate number of children from ethnic minority backgrounds who are detained. Although these children comprise just under a quarter (23%) of all child arrests, they make up more than half (54%) of children remanded into custody.

The UK has signed up to the UN principle that children should be detained only as a last resort. But the report describes how many are treated like adults: held in police and court cells, delivered to court in secure vans, accompanied by security guards and placed in secure docks.

It adds: “Their remand hearing is sometimes presided over by magistrates with no youth training, bail is opposed by non-specialist prosecutors, and the law applied mirrors adult remand law. And a lack of suitable accommodation means alternatives such as remand to local authority are woefully underused.”

“Under-18s should be dealt with by the youth court system,” said the report’s co-author, Penelope Gibbs. “But many slip through the cracks.”

Writing in the foreword to the report, Dr Laura Janes, legal director at the Howard League for Penal Reform, says: “Custody is... much more than a deprivation of liberty: for many it can be a deeply traumatic experience, and on occasion, such as in the tragic suicide of William Lindsay, 16, at Polmont in Scotland, it can be a death sentence.”

The Scotsman reported that the vulnerable teenager, who had been in and out of care, killed himself at a young offender institution within 48 hours of being remanded there in October, despite having been flagged as a suicide risk.

Youth offending teams (YOT) told Transform Justice they believed the rise in remand may be due to a “more punitive and risk-averse” reaction to knife crime. The number of offences involving possession of a knife or offensive weapon committed by children has increased 11% since 2012, while the number committed by adults has dropped 10% over the same period.

Child prisons are beyond reform - it's time to stop jailing young people | Carolyne Willow Read more

Almost half of those remanded (45%) are awaiting trial for violence against another person. Just over a fifth (21%) are on remand for robbery.

“Remand becomes particularly likely if the child is said by the prosecution to be involved in a gang, particularly given that the government’s definition of gang-related violence is broad,” the report states.

Children involved in the “county lines” drug trade who are arrested far away from home form a significant proportion of those being remanded.

“I can’t speak for the other courts, but I think if you’re from the city and you end up in some shire, and they’re not used to it, they might see these young people as some kind of gangsters,” one YOT member told Transform Justice. “I don’t think they see that behind it they might be criminally exploited, or vulnerable. They just see the impact of this visitor to their county selling drugs on a large scale so they might be quite strict.”

A spokesman for the judiciary said a working group was examining the issue of youth remand and that judges received guidance on appropriate use. “All magistrates and district judges who sit on youth cases are trained in relation to all aspects of practice and procedure in relation to children and young people,” he said.