Support from senior officers comes as high court is due to issue judgment about rules on under-18s in police custody

Senior police officers are backing a change to the law so that 17-year-olds are treated as juveniles in custody and their parents are informed if they are arrested.

Support from the Association of Chief Police Officers (Acpo) for parental involvement comes as the high court issues a key judgment on Thursday on whether all those under 18 should be treated as children – in line with the UN convention on the rights of the child.

Ann Thornber, whose son Ed killed himself in 2011 after being arrested by police, will be in court to hear judgment. She has delivered a petition to Downing Street calling for a change in the law – along with the parents of Joe Lawton, another 17-year-old who took his life after being arrested and breathalysed.

Thornber, from Manchester, told the Guardian that her son had been on holiday in Cornwall with friends in the summer of 2011 when he was detained for possession of 50p's worth of cannabis.

"He was told he would get a formal warning," she recalled. "Because he lived in Manchester, it was going to be issued by Manchester police. We were not told he had been arrested.

"On 13 September 2011, he was [mistakenly] served with a court summons. When my husband and I came home from work, Ed didn't tell us anything.

"The next day, he went out to some woods nearby and hanged himself. He didn't know the summons had been issued by mistake. We didn't know anything about it.

"We have seen a transcript of the police interview [in Cornwall]. Ed was asked if he wanted a solicitor. He wouldn't know what one was for. If the law had bee different, we would have found out what had happened and been able to support him.

"But we didn't know anything about it. If they had changed that rule, he would be alive today."

The chief constable of Cleveland police, Jacqui Cheer, this week acknowledged in a letter to the coroner who conducted the inquest into Thornber's death, that the law was inconsistent. Cheer, who is the Acpo lead officer on issues involving children and young people, was responding to a request under rule 43 of coroners' rules – a mechanism used to highlight safety matters of broader public interest.

She wrote: "The position of 17-year-olds as adults under Pace [the Police and Criminal Evidence Act] is not consistent with the way in which they are treated under other legislation, for example, the Children's Act 1989 and the Legal Aid Sentencing and Punishment of Offenders Act 2012 not to mention the UN convention on the rights of the child.

"We find this incongruous and would support a change in legislation to bring Pace into line with other legislation and for 17-year-olds to be treated as juveniles when in police custody.

"However, this would require a change in legislation and is therefore properly a matter for government. … We do not feel it would appropriate for the police service to voluntarily act in contravention of legislation."

The judicial review case challenging Home Office policy on how to treat 17-year-olds in custody has been brought by the charity Just for Kids Law. It argues that 17-year-olds do not know what is in their best interest and should routinely be provided with the support of a parent or appropriate adult when in custody.

Ann Thornber said that her son was a keen lacrosse player and he believed that a criminal conviction would destroy his life chances.

"He had had trials for England and wanted to go to the States to play and coach. He knew that if he had a criminal conviction he wouldn't have been able to get a visa … One of the fundamental roles of being a parent is that your are there to support your child all the time and we couldn't fulfil that role. We didn't know what had happened.

"A change to the law won't bring Edward back but it would prevent others going through this. It was an unnecessary loss because it should never have happened."

During the court hearing it was said that the government's refusal to reschedule 17-year-olds as children was down to the costs involved. The high court court heard that it could add an extra £21m to policing bills.

The Home Office maintains that the UN convention on the rights of the child is not given effect by any domestic legislation and "does not form part of English law".