A single particle of gunshot residue is at the centre of an appeal by the man found guilty of murdering BBC presenter Jill Dando, senior judges have heard.
Barry George has always denied he murdered Miss Dando
It is the second attempt by Barry George, 47, to overturn his conviction.
His lawyer, William Clegg, told the Court of Appeal George's conviction was unsafe because fresh evidence "neutralised" the firearms evidence.
George is serving life for shooting Miss Dando dead outside her west London home in 1999.
If the appeal judges decide the conviction is unsafe, they could order a retrial.
Mr Clegg said during a search of George's flat nearly a year after the shooting, police had seized a Cecil Gee coat.
One particle of firearm discharge residue found to be consistent with being produced by the fatal shot had been found on the lining of an inside top pocket, Mr Clegg said.
"Although there was clearly other important evidence in addition to the FDR [firearm discharge residue], the effect of neutralising the FDR evidence must be to render the conviction unsafe," he told the court.
JILL DANDO MURDER CASE
26 April 1999 Jill Dando is shot dead in west London
25 May 2000 Barry George, also known as Barry Bulsara, is arrested
29 May 2000 George is charged with murder
2 July 2001 George is found guilty of murder and later sentenced to life in prison
29 July 2002 George loses an appeal against his "unsafe" conviction
16 Dec 2002 House of Lords refuses permission for further challenge
20 June 2007 The Criminal Cases Review Commission grants George the right to another appeal
Mr Clegg said the original trial in 2001 had been misled and called for George to be tried before a jury for a second time.
He argued the firearm evidence carried "zero or neutral evidential weight" and if it was admitted again before a jury, it should have the proper weight attached.
Mr Clegg said after the verdict there had been concern by some at the Forensic Science Service (FSS) that the firearms evidence might have been given undue weight when placed before the jury.
Dr Ian Evett, an FSS employee since 1966, said his position was one of "vague unease", Mr Clegg said.
He said an FSS report found "it would be just as likely that a single particle of discharge residue would have been recovered from his pocket whether or not he was the person who shot Miss Dando nearly a year previously".
George was in court for the opening of what is expected to be a three-day hearing.
Dressed in a blue shirt and dark jacket, he spoke only to confirm his name.
Mr Clegg told the court a clinical psychologist would help George to follow proceedings as he had learning difficulties.
Lord Phillips said neither he nor the other three judges had seen either of the recent TV documentaries about the case.
After the appeal, the court would consider the propriety of those broadcasts, he said.
Outside court, George's sister, Michelle Disken, said: "We're very grateful to them that we can actually be back here today - and today we're hoping that we're going to get justice at last for Barry."
George was found guilty by a majority verdict of 10 to one after the Old Bailey jury heard forensic scientists had found a single speck of residue from the gun used to kill Miss Dando, 37, in the pocket of his coat.
He lost his first appeal a year later.
When granting the second appeal in June of this year, the Criminal Cases Review Commission said too much emphasis had been placed on the speck of firearm discharge at George's trial.
Described as the "golden girl of television", Miss Dando presented BBC programmes including the Six O'Clock News, Breakfast News, Crimewatch and Holiday.