Lawyers acting for the man convicted of murdering Lin and Megan Russell in England in 1996 say they have new evidence of his innocence, including a "very detailed confession" to the killings from serial killer Levi Bellfield.
Michael Stone has twice been convicted of the murders of the mother and daughter in Kent, but has always protested his innocence.
Now his solicitor Paul Bacon has claimed Milly Dowler's murderer Bellfield admitted the killings in prison.
He told a press conference in London: "We have now received evidence of a full confession by Levi Bellfield to the Russell murders.
"In the confession, Bellfield describes how he came across Lin Russell and her two children, how he attacked them with a hammer and his motivation for the killing.
"The confession is detailed and has a number of facts which are not in the public domain."
Stone is serving three life sentences for bludgeoning Lin Russell and Megan, six, to death in an attack near Chillenden in which her other daughter, Josie, nine, suffered severe head injuries and the family's dog, Lucy, was killed.
It is not the first time Stone's legal representatives have pointed the finger at Bellfield.
In 2011, Mr Bacon claimed Milly's killer was a better match for the e-fit of the suspect issued during the police manhunt.
Bellfield, who now calls himself Yusuf Rahim, is serving a whole-life tariff after being found guilty at the Old Bailey of abducting and killing schoolgirl Milly. He was jailed in 2011.
The 13-year-old was snatched from the street while on her way home from school in Walton-on-Thames, Surrey, in March 2002.
Bellfield was already in jail for the murders of Amelie Delagrange and Marsha McDonnell, and the attempted murder of Kate Sheedy.
In 2008 he had been given a whole-life term for murdering Ms McDonnell, 19, in 2003, and murdering Ms Delagrange, 22, and attempting to murder Ms Sheedy, 18, in 2004.
Stone was told in 2006 that he must spend a minimum of 25 years behind bars over the attack on Dr Russell, 45, and her daughters as they walked home from a swimming gala along a country lane in July 1996.
Josie survived her injuries and went on to carve out a new life as an artist, growing up with her father, Dr Shaun Russell, in Wales.
In November last year, the Metropolitan Police closed a probe into serious crimes allegedly involving Bellfield, saying all lines of inquiry had been "exhausted" and officers had found no evidence linking him to cases for which he had not already been convicted.
In October 1998, a jury at Maidstone Crown Court found heroin addict Stone guilty of counts of murder and attempted murder.
An appeal over doubts about a key witness saw the convictions quashed by Court of Appeal judges in 2001, before Stone was convicted again at a second trial at Nottingham Crown Court and, in October 2001, was given three life sentences.
Stone was convicted on the evidence of Damien Daley, who admitted in the witness box that he had lied about his drug-taking exploits at the first trial in 1998.
Daley said Stone confessed to the hammer murders through a heating pipe into the next cell at Canterbury Prison.
There was no forensic evidence against Stone, who pleaded not guilty, and maintained throughout that his cellmate was lying about his confession.