A brain damage patient who was jailed for a murder he insists he did not commit has had his conviction quashed and been told he will not have to stand trial again.

Oliver Campbell, who lived in Stratford at the time of his arrest, has been fighting for more than 30 years to quash his conviction.

He originally confessed to the fatal Hackney shooting of shopkeeper Baldev Hoondle on July 22, 1990.

But a three-day Court of Appeal hearing earlier this year heard from experts that Mr Campbell had the mental age of a seven-year-old and there was a high chance he had falsely confessed under “extreme” pressure from police.

Two young, Black men had robbed the G and H Supermarket in Lower Clapton Road on the night of the shooting. Mr Hoondle was shot dead at point blank range during a scuffle with one of the robbers.

As the culprits fled, one of them dropped a baseball cap in the street which said “British Knights” on it.

The Metropolitan Police Service deemed Mr Campbell a suspect because he had bought one of the distinctive hats prior to the robbery.

However, he did not fit eyewitnesses’ descriptions of the shooter. They all agreed that the shooter was under six foot and an older man, whereas Mr Campbell was 6’3” and 19.

Mr Campbell was put in three eyewitness line-ups and none of the witnesses picked him out – including Mr Hoondle’s son, who had looked the shooter right in the face inside the shop.

But six weeks after the ID parades, one of the witnesses changed his mind and said Mr Campbell was the shooter.

Police arrested Mr Campbell, who had suffered severe brain damage as a baby, and subjected him to what one expert witness called “extreme” pressure.

Officers were persistently rude to Mr Campbell, accused him of lying when he denied the offence, and lied to him, falsely claiming they had forensic evidence proving he was the killer.

In fact, hairs found inside the baseball cap did not match Mr Campbell.

He eventually made confessions, which barrister Michael Birnbaum KC described at the Court of Appeal in February as “a tissue of nonsense… contrary to the known facts”.

For example, Mr Hoondle was shot in the left side of his head at point blank range and had been grappling face-to-face with the robber, meaning he must have been shot with the robber’s right hand – but Mr Campbell’s brain damage means he has almost no use of his right hand.

However, a jury convicted him in 1991. He spent 11 years in prison for the crime.

“The wheels of justice move very slow,” he said outside the Court of Appeal in May.

In 2002, the BBC aired a Rough Justice documentary about Mr Campbell’s case, which revealed that his co-defendant Eric Samuels – who admitted involvement in the robbery – had told police from the start that Mr Campbell was innocent and his real accomplice was a man called Harvey.

He told police he and Harvey had stolen Oliver’s baseball cap hours before the robbery.

But after making this admission in the back of a police car, he took legal advice and refused to repeat it in any formal statement, interview or court testimony.

It would be another 20 years before the Criminal Cases Review Commission (CCRC) – the official body for helping to overturn miscarriages of justice – finally referred his case to the Court of Appeal.

It did so on the basis that scientific advances had shed more light on Mr Campbell’s mental impairment and the possibility that his confessions had been false.

In a judgment published this morning, Court of Appeal judges wrote: “We accepted that the fresh expert evidence… adds material information about the risk of false confession which was not and could not be known at the time.

“It follows that the conduct of the trial would have been materially different if that information had been known at the time.”

They added: “We are acutely conscious that our overall decision will be a heavy blow to the bereaved family and friends of Mr Hoondle.

“They have behaved with great dignity during the hearing of the appeal. We sympathise with them in their loss and trust that they will understand that we must reach our decisions in accordance with the law, uninfluenced by emotion.”