Jeremy Bamber's convictions for murdering five of his relatives more than 25 years ago will not be referred to the Court of Appeal, officials said on Thursday.
The notorious inmate, serving a whole life term for the 1985 killings, has always protested his innocence and claims his schizophrenic sister Sheila Caffell shot her family before turning the gun on herself in a remote Essex farmhouse.
The Criminal Cases Review Commission said that despite a lengthy and complex investigation, it "has not identified any evidence or legal argument that it considers capable of raising a real possibility that the Court of Appeal would quash the convictions".
The Commission said this was its final decision in its longest-running case.
Giving its reasons in a 109-page statement, it said: "Matters of pure speculation or unsubstantiated allegation constitute neither new evidence nor new argument capable of giving rise to a real possibility that the Court of Appeal will quash a conviction.
"Neither can such a real possibility arise from the accumulation of multiple unsubstantiated allegations.
"The Commission is satisfied that nothing in the submissions made by and on behalf of Mr Bamber or any issues raised in the recent documentary can, either individually or cumulatively, give rise to a real possibility that the Court of Appeal would find any of Mr Bamber's convictions to be unsafe."
Jeremy Bamber in 2002
Mr Bamber was told of the decision in prison today, the Commission confirmed.
A spokesman added: "This is a final decision and brings to a close the Commission's current longest running case.
"The Commission has given due consideration to all the submissions made, old and new, before making a final decision on whether to refer the case to the Court of Appeal."
Bamber, 51, who is being held in Full Sutton prison in York, has been behind bars for 25 years for shooting his wealthy adopted parents, June and Neville, his sister Ms Caffell and her six-year-old twin sons Daniel and Nicholas at their farmhouse in Tolleshunt D'Arcy, Essex, on 7 August, 1985.
He was given a whole life tariff after being convicted of the murders in October 1986.
In 2009 Bamber lost a Court of Appeal challenge against the order that he must die behind bars. He has twice lost appeals against conviction.
Bamber's lawyer Simon McKay said his client was "very disappointed" and considering applying for a judicial review of the Commission's decision.
"He is obviously very disappointed but remains determined to carry on the fight to clear his name," Mr McKay said.
"In my view the Commission have not applied the proper test for determining whether a case should be referred back to the Court of Appeal.
"To be clear: four independent and supremely qualified experts provided opinions that fundamentally undermined the Crown case against Mr Bamber and the safety of the convictions.
"The evidence was credible, inherently believable and gave rise to cogent admissible grounds of appeal that may have affected the jury's verdict.
"This is sufficient for the case to be referred back: whether the conviction is in fact subsequently quashed is a matter for the Court of Appeal. The Commission may have usurped the court's function."
He went on: "I will now be considering a judicial review of the Commission's decision.
"In the final analysis, whatever the notoriety that surrounds this particular case, whatever the public perception, the law applies equally to Jeremy Bamber as it does to all of us.
"The fight to clear his name will endure until justice prevails."