A killer who has served a decade in jail for murdering a prison officer is to fight his conviction after an appeals court heard he was found guilty off the back of evidence from an unreliable witness.

Mark Dorling, 48, was jailed for life in 2006 after being found guilty of the murder of Aaron Chapman, 23, who was stabbed at his home in Sutton in December 2002.

The savage murder was alleged to have been a ‘contract killing’ carried out amid a murky gangland war.

But nightclub bouncer Dorling, himself a former prison officer who was given the nickname ‘Robocop’ by his colleagues, is now appealing his conviction after evidence emerged that is said to destroy the credibility of a key prosecution witness.

A lawyer who gave evidence of Dorling’s alleged confession has since come under strong criticism from judges in other cases, Lord Justice Gross told the Court of Appeal on Friday.

This later resulted in a court ruling that the man, who is in witness protection, ‘could not be relied on as a witness of truth’.

Following the revelations, the Criminal Cases Review Commission (CCRC) referred Dorling’s case to the Court of Appeal.

During the 2006 trial a judge heard how Dorling had been sent to Mr Chapman’s house in Goosens Close with orders for a ‘punishment hit’ that stopped short of actually killing him.

But High Down prison officer Mr Chapman is believed to have fought back, with fatal consequences – he died after being stabbed a total of 16 times, including in his heart and liver.

At Dorling’s trial the Old Bailey heard how Mr Chapman was a notorious womaniser who dealt cocaine and was suspended from his job for moonlighting as a bouncer in Cheam and Croydon.

On Friday Lord Justice Gross said: “There was a real possibility that this court can no longer be satisfied as to the safety of the conviction.”

He added: “Mr Dorling has a strong and legitimate interest in this being heard without delay.”

Judges ordered the prosecution to complete work on disclosing evidence within a month, but it is unlikely that the appeal will be heard before the autumn.

Dorling, of Thornton Heath, is serving life with a minimum of 25 years before he is eligible for parole.