A mentally ill and violent teenager who threw a six-year-old boy from the Tate Modern viewing gantry when he was allowed out unsupervised has been told he “may never be released” after being jailed for at least 15 years for attempted murder.

Jonty Bravery, now 18, was said to have had “a big smile on his face” when he was challenged by horrified onlookers – including the victim’s distraught and disbelieving parents – moments after hurling the young tourist over railings.

The victim survived the 100ft (30m) fall, but suffered life-changing injuries – including a bleed on the brain and multiple broken bones – and remains in a wheelchair. He will require round-the-clock care support until at least 2022.

Sentencing Bravery, of Ealing in west London, Old Bailey judge Mrs Justice McGowan said: “The fear he (the victim) must have experienced and the horror his parents felt are beyond imagination.

“You had intended to kill someone that day – you almost killed that six-year-old boy.”

She said Bravery’s autism spectrum disorder (ASD) did not explain the attack, and acknowledged expert evidence he presents “a grave and immediate risk to the public”.

The judge added: “You will spend the greater part – if not all – of your life detained … you may never be released.”

Well-built Bravery, who was wearing a white T-shirt and dark shorts, sat impassively with his legs crossed and occasionally placed his hands behind his head as he watched the 20-minute hearing via videolink from Broadmoor Hospital.

The court heard Bravery had been in supported accommodation under the care of Hammersmith and Fulham Social Services, with one-to-one supervision, and had a history of lashing out at staff.

Despite this, he was allowed to leave home, unsupervised, for up to four hours at a time.

Prosecutor Deanna Heer said there was evidence Bravery had long harboured his intent to seriously hurt or kill someone, with the teenager’s admissions apparently caught on a “shocking, prophetic” secret recording made by carers. The alarm was not raised with Bravery’s parents.

It was on Sunday August 4 2019 that Bravery – who has a mental disorder – left his accommodation and travelled to the Tate Modern in central London, spending at least 15 minutes stalking potential victims before “scooping” a six-year-old boy up and over the railings as the youngster skipped slightly ahead of his family.

CCTV footage not shown in court captured the incident, then showed Bravery backing away from the railings.

The prosecutor said: “He can be seen to be smiling, with his arms raised. At one point, he appears to shrug and laugh.”

Ms Heer told the court Bravery then told the boy’s father: “Yes I am mad.”

He was also heard to say, with a shrug: “It’s not my fault, it’s social services’ fault,” the lawyer said.

It later emerged that Bravery initially sought to carry out his grim attack at the Shard, Britain’s tallest building, but baulked at the entry fee.

Following his arrest, Bravery was said to have asked police if he was going to be “on the news”.

He said he had been “seriously unhappy” recently and that he had to do anything he could to get out of his accommodation.

Bravery admitted attempted murder at the Old Bailey last December.

Ms Heer told the court: “He said he had to prove a point to ‘every idiot’ who had ever said he did not have a mental health problem that he should not be in the community.”

Bravery later disclosed to a psychiatrist that he planned the offence well in advance and researched the easiest way to kill someone, narrowing it down to three possibilities – strangling a woman or a child, drowning a child, or throwing someone off a tall building.

Defence counsel Philippa McAtasney QC said her client was immature, and said it “beggars belief” that he was deemed suitable to go out unsupervised.

She said Bravery’s parent’s “abhor” what he did and cannot forgive him, but feel “let down by the system”.

In a victim impact statement taken in February, the boy’s parents described Bravery’s actions as “unspeakable”.

The couple, who have now returned with their son to their native France, said: “Words cannot express the horror and fear his actions have brought up on us and our son who now, six months on, is wondering why he’s in hospital.

“How can he not see in every stranger a potential ‘villain’ who could cause him immense pain and suffering?”

No members of the victim’s or Bravery’s family were present in court for the sentencing.

A Hammersmith & Fulham Council spokesman said a serious case review was under way, and they were co-operating fully and would learn from its findings.

He said: “We wish to extend our sincere sympathies to the young child and his family after the terrible event at Tate Modern in August 2019. We have been pleased to learn of the progress the child is making.”