Fraser claims he was denied a fair trial because the jury knew of a previous conviction for assaulting his wife Arlene in 2000.
His lawyers have interviewed lip reader Jessica Rees over claims that she saw a juror say: "I told you so" when judge Lord Mackay revealed Fraser, 44, had served time for the attack.
One Grampian Police source said: "This is potentially dynamite but Fraser's lawyers have been kept very tight.
"They had already been making a lot of noise about pre-trial publicity affecting Fraser's case and this has only added fuel to that."
Lawyers acting for Fraser have travelled to England to quiz expert witness Jessica, whose identity we have chosen to protect, about what she heard and have also spoken to several Grampian Police officers.
The police source said: "This woman helped put Fraser behind bars where he belongs now she could inadvertently help him cheat justice."
Jessica, 35, who learned to lip-read after meningitis left her deaf aged four, had been recruited by the prosecution to decipher video-taped conversations between Fraser and his friend Glenn Lucas.
The video images, filmed in Porterfield Prison, Inverness, had no sound but Jessica claimed the men discussed the disposal of Arlene's body in gory detail. She had been cited as a key witness but was not called to give evidence after former co- accused Hector Dick agreed to testify against Fraser.
But Jessica was in court for the verdict - and what she saw may have thrown Fraser a legal lifeline.
The remark allegedly made by a juror will be used by his lawyers to appeal his conviction of the notorious killing.
They will argue that publicity before the court case made a fair trial impossible and that members of the jury may have been biased against Fraser because they knew he had previously been convicted of attacking Arlene.
A key part of the grounds of appeal will be what Jessica saw in court minutes after the jury delivered their guilty verdict.
She is thought to have witnessed a conversation between two jurors minutes after Fraser was sentenced at the High Court in Edinburgh.
The jury returned a majority verdict of guilty against Fraser at the High Court in Edinburgh in January.
Trial judge Lord Mackay then described Fraser as evil before sentencing him to a minimum of 25 years in prison.
The jury then heard that Fraser had previously been jailed for 18 months for assaulting Arlene a month before her disappearance.
It was at this crucial moment that Jessica happened to focus on a conversation between two jurors.
She later told a police officer sitting beside her during the trial what she had witnessed.
One suggestion is that after the judge revealed Fraser's previous conviction, one juror turned to another and said: "See, I told you."
"Silent witness" Jessica was recruited to decipher video-taped conversations between Fraser and Glenn Lucas, 51.
Lucas was originally charged with conspiring with Fraser and Dick to murder Arlene but the charges were later dropped.
He was filmed visiting Fraser at Porterfield Prison in 2001, where he was serving time for fiddling a legal aid application. Mum-of-three Jessica refused to confirm details of the courtroom incident when approached by the Sunday Mail and referred all further enquiries to the police.
When the jurors were sworn in they would have been told to disregard anything they had previously read or heard about Fraser.
They would have been told that if they could not accept those instructions they could not sit on the jury.
Jessica has been used by police forces across Britain to crack a host of difficult cases.
She can lip read even when her subject is not facing her, is speaking in an accent and has a beard.
She helped snare two IRA bombers as well as the Heathrow Airport gang, who she saw discussing how they would spend the money.
Fraser's lawyers had objected to her being called as a witness claiming the "invasion of privacy" breached the European Convention on Human Rights.
The issue was resolved when Dick turned Queen's evidence and the lip reader wasn't needed by the prosecution.