LONDON (AFP).- A British teenager who threw a young French boy from the viewing platform of London's Tate Modern gallery failed in a legal bid on Wednesday to reduce the length of his jail term.

Lawyers for Jonty Bravery had argued in the Court of Appeal in central London that the 15-year minimum of a life sentence for attempted murder was too high.

But three judges disagreed, ruling that it was not "manifestly excessive or wrong in principle".

Bravery, 19, was jailed in June for throwing the young boy head first from the 10th-floor balcony of the London gallery onto a fifth-floor roof 30 metres (100 feet) below.

The boy, who cannot be identified because of his age, broke his spine, leg and arms and suffered a head injury.

The trial judge said he had suffered "permanent and life-changing injuries". She told Bravery he was a "grave danger to the public" and may never be released.

Bravery, who was 17 at the time of the offence, brought the appeal to secure a transfer from prison and instead be subject to a hospital order because of his psychological issues.

The teenager said after his arrest that he had carried out the attack because he had not been given proper treatment for mental health problems.

He was diagnosed with Autism Spectrum Disorder at the age of five and has a personality disorder. Psychiatrists at his trial also said he may have psychopathic traits.

His lawyers abandoned an attempt to secure a hospital order after a consultant psychiatrist said he appeared to have settled into the prison system.

Prosecutors defended the sentence, saying it was justified because the offence, which happened in full view of the boy's parents and a horrified crowd, was "of exceptional callousness".

Bravery's young victim was on holiday at the time of the attack in August last year and still requires around-the-clock care and rehabilitation.

© Agence France-Presse