A six-year-old French boy who was thrown from a viewing platform at London's Tate Modern art gallery may never fully recover from his injuries, a court sentencing hearing for his attacker was told on Thursday.

Jonty Bravery, 18, has admitted trying to kill the child at the riverside tourist attraction on August 4 last year, in front of a crowd of horrified visitors.

His victim, who was visiting London with his family at the time, broke his spine, legs and arms as he fell on to a fifth-floor roof below.

A judge at the Central Criminal Court, the Old Bailey, was told he spent more than one month in hospital in Britain before being discharged for further treatment in France.

He suffered a significant brain injury, remains in a wheelchair and will require round-the-clock care until at least August 2022.

"Whether he will ever make a full recovery is not known," prosecutor Deanna Heer said.

The boy was "fortunate not to die" and the attack was "a whisper away from murder", she added.

Bravery, wearing a white t-shirt and dark shorts, followed proceedings via videolink from Broadmoor high-security psychiatric hospital in southern England.

At one point he was seen to crouch on the floor and pulled his t-shirt over his head. At other times he appeared to be asleep.

The court was told that medics appeared to agree Bravery, who has pleaded guilty to attempted murder, has Autism Spectrum Disorder and a personality disorder.

Heer said a psychiatric report indicated his "callousness and striking lack of emotional empathy... is not typical of autism but is more typically found in psychopathy".

"The defendant fully understood the consequences of his actions and was capable of exercising self-control to resist acting in the way that he did," she added, citing the report.

At the time of the attack, Bravery was 17 and living in supported accommodation and under one-on-one supervision but was allowed out unaccompanied for short periods.

The fall left the young boy, who was visiting London with his family at the time of the attack, with a broken spine, legs and arms
The fall left the young boy, who was visiting London with his family at the time of the attack, with a broken spine, legs and arms AFP / Daniel SORABJI

On the day, he was seen wandering about the Tate's viewing gantry moments before he "scooped up" the young boy and threw him head first over the railings.

The court was told the attack was planned and he wanted to prove he had a mental health condition and was not getting proper treatment.

Heer said Bravery was smiling, and laughed and shrugged when challenged about what he had done. But he said: "Yes, I am mad... It's not my fault. It's social services' fault".

"He said he wanted to be on the news so that everyone, especially his parents, could see what a mistake they had made by not putting him in hospital," she added.

Judge Maura McGowan was told the minimum sentence in such a case was 20 years in custody.

But consultant forensic psychiatrist Joanna Dow recommended Bravery should be detained in hospital and it was "hard to envisage" he would ever be released into the community.

McGowan said she would not pass sentence until Friday morning.

The young boy's parents said in a victim impact statement they had barely left their son's side, and described the attack as "unspeakable".

"We have been so scared of losing him that now it is impossible for us to spend more than a few hours away from him," they added.

Their son, who cannot be identified because of his age, now found it hard to trust anyone, they said.

The couple have separately issued statements about his recovery through a GoFundMe page to raise money for his care. It has so far raised almost 234,000 euros ($260,000).

In May, they said he still struggled with speech, eating and drinking, and movement, although he could stand and take a few steps with the aid of a walker.

"There is still a long way to go but we are holding on, even confined and masked" because of the coronavirus outbreak, they said.