Papers have shown a gun at the centre of a 1973 murder trial may have been the wrong weapon, it has been claimed.
Paul Cleeland, who says he was wrongly convicted, has obtained a transcript of his failed appeal in 2002.
He said prosecutors claimed Terry Clarke was shot in Hertfordshire in 1972 with a Gye and Moncrieff (G&M) but two other guns were found at the time.
His lawyers have said judges wrongly ruled the other guns were excluded. A court will hear the claims on Tuesday.
Suspected gangland boss Mr Clarke, a friend of Cleeland’s, was shot in Stevenage after returning home from a bar on 5 November 1972.
Cleeland always insisted he was innocent and was at home with his wife when Mr Clarke was killed but he served 26 years behind bars.
Court documents listed the guns as the G&M shotgun, an American Western Field 12-bore pump action repeater shotgun and a German Muller 12-bore double-barrelled shotgun.
A note by Edward Fitzgerald QC, who previously represented Cleeland, claimed the Appeal Court ruling in 2002 “was flawed by a fundamental error of fact”.
He wrote: “The Court of Appeal in 2002 proceeded on the basis that the evidence excluded the possibility that either of the two shotguns put forward by Mr Cleeland as the murder weapons could have been the murder weapons.
“However, the transcript of the evidence obtained by Mr Cleeland confirms that the Home Office expert, Mr Pryor, did not rule out the possibility that the pump action shotgun was the murder weapon.”
Lawyers for the 79-year-old from Folkestone, who has been representing himself, obtained the transcript after accessing a closed file in the National Archives.
Folkestone MP Damian Collins, who has backed Cleeland’s campaign to clear his name, said: “Over many years, Paul was unaware of the contents of those papers when they could have real significance to his case.”
The MP said most of the original trial evidence had been challenged and there was “considerable doubt” over the weapon, cartridges found at the scene, the type of weapon used, and lead residue that could have had other environmental sources.
He said: “Over the past 10 years I’ve been involved with it [Cleeland’s case], a constant stream of new information has come out and I would say the direction of that information has always been to cast doubt on that original judgement.”
Last year, Cleeland sought a judicial review of a decision by the Criminal Cases Review Commission (CCRC) not to send his case to appeal but his challenge was dismissed.
The CCRC has not commented on Cleeland’s latest action.