One of the witnesses, who both told the jury that Stone confessed to the killings while in prison, said they had been in the pay of Kent Police. The allegation comes only days before Stone begins his appeal against conviction.
Barry Thompson was the witness who told Maidstone Crown Court back in 1998 that Stone had described his failure to kill Megan's sister Josie as a 'mistake'.
It is Thompson who has now told the Daily Mail in a recorded conversation that he was an informer at the time. He said that fellow informer Damien Daley was also regularly used by police.
And he expressed fears that this information would be suppressed during Stone's appeal next month.
Last night Stone's solicitor Derek Hayward said the revelation could be a vital breakthrough. 'This information is potentially of crucial importance in the case and we will certainly be raising these issues at the appeal hearing,' he said.
The dramatic development follows a series of articles in the Daily Mail over the last two years casting doubt on the life sentence imposed on Stone in October 1998 for the murders of Dr Russell and six-year-old Megan, who were bludgeoned over the head with a hammer as they walked home through the Kent countryside in July 1996.
Nine-year-old Josie was also attacked and suffered massive head injuries but made a recovery, and today lives with her father, Dr Shaun Russell, in North Wales.
Last night Kent Police refused to discuss the new claims. They centre on confessions that were allegedly heard by Daley and Thompson from Stone while he was on remand in prison awaiting trial.
Stone, a petty thief and drug addict, has always maintained the confessions were bogus.
Thompson, 35, who has been convicted of deception and intimidating witnesses, told the murder trial that Stone had threatened him at Emley jail, and had said he had made a 'mistake' in not killing Josie.
But just days after the verdict Thompson told a national newspaper that his testimony had been a pack of lies.
Daley, 25, was said to have met Stone in Canterbury prison.
He said that the pair spoke to each other through a cracked drainpipe and told the court that Stone had described the Russells as 'whores and bitches' and how the murder had sexually aroused him.
Another prison witness, Mark Jennings, also told the jury that he heard a confession from Stone. But it later emerged that his family been paid £5,000 in advance of the verdict by a national newspaper with a promise of a further £10,000 if Stone was found guilty.
When asked by the Mail why he stopped being an informer, Thompson said: 'It's Stone. I didn't believe it in the first place. I never thought he'd done it. The evidence proves it. There was nothing to point he'd done it. Nothing at all.'
Stone's supporters have pointed to a lack of forensic evidence and eyewitness accounts placing him at the murder scene. They claim that hairs from Josie's beach shoes and red fibres found on her mother's body could not be linked to Stone.
Neither was blood from any of the Russells found on him.
Last night a third man who made statements to the Russell murder squad also said he was an unpaid police informer.
The man, known as Prisoner X, told detectives that Stone had confessed to the murders while remanded on the hospital wing at Belmarsh Prison in August 1997.
'I was registered as an informer at Romford police station in 1989,' he said. 'My name was Top Gun.'
Prisoner X was nearing the end of a two-and-a-half year sentence for deception when he met Stone. He alleges that on his day of release he was taken to the office of the prison hospital wing by a prison officer.
'She had two SIR sheets (Security Information Reports), which were covered in writing,' said X. She asked "Is this a true account of your conversations with Michael Stone?" 'I said "Yes". I didn't bother to read them, I just signed them.'
As a result of his cooperation, X claims that outstanding charges against him were then dropped. In the event he was never called as a witness.