Jeremy Bamber is bringing a High Court bid for evidence to be released which he claims could lead to his convictions for the murders of five family members more than 30 years ago being overturned.
The 59-year-old is serving life behind bars after being found guilty ofmurdering his adoptive parents Nevill and June, both 61, his sister SheilaCaffell, 26, and her six-year-old twins Daniel and Nicholas at White House Farm, Essex, in August 1985.
He has always protested his innocence and claims Ms Caffell, who suffered from schizophrenia, shot her family before turning the gun on herself.
Bamber's lawyers claim the Crown Prosecution Service (CPS) has not disclosed material about a second silencer which is said to have been found at White House Farm, which they argue is relevant to his latest attempt to overturn his conviction.
Joe Stone QC, representing Bamber, told a remote High Court hearing on Friday that "it now seems almost certain that there is a second sound moderator" evidence he suggested could "significantly undermine the prosecution case".
He told Mr Justice Julian Knowles the second silencer "could be the mostimportant exhibit in the case - we simply do not know its true relevance".
Mr Stone said:
"The prosecution case at trial was based exclusively on the point that there was only ever one sound moderator and that the specific factual circumstances in which this sound moderator was found and the nature of the blood grouping on it meant that Sheila Caffell could not have committed the murders and then taken her life."
The court heard the prosecution case at Bamber's trial in 1986 was that MsCaffell could not have reached the trigger to kill herself if the silencer wasattached to the murder weapon.
Mr Stone argued his client would be "significantly handicapped" in mounting a fresh bid to overturn his conviction through the Criminal Cases ReviewCommission (CCRC), the independent body which investigates possible miscarriages of justice, without the evidence sought.
He added that "blood-based exhibits from Sheila Caffell's DNA were destroyed by Essex Police in February 1996", as part of what he described as "systematic destruction of core exhibits".
Mr Stone also referred to a recent ITV drama about the killings, White HouseFarm, and claimed that material was provided to the author of a book on which the show was based by the original investigating officer "without permission or lawful authority".
He said author Carol Ann Lee had been given "a number of sensitive casedocuments" by former detective superintendent Michael Ainslee, who is said to have later destroyed the items despite knowing "there were ongoing appeal proceedings".
Mr Stone said: "This week, the claimant (Bamber) was informed that theoriginal senior investigating officer Michael Ainslee had in fact taken back tohis home address, in 2010, more documents from the murder investigation, and had in fact destroyed them, and it is not known what these particular documents were."
Annabel Darlow QC, for the CPS, said in written submissions: "The allegedexistence of two sound moderators, and the integrity of the exhibits relating to the examination of the sound moderator, have been the subject of exhaustive inquiry and review."
She said the CPS "does not accept that the documents now sought were notpreviously disclosed or made available to the claimant".
She added: "The relevant evidence at trial was the existence of one soundmoderator, contaminated with deposits explicable only on the basis that it was attached to the firearm used for the murders.
"Even if, contrary to the overwhelming weight of the evidence, the existenceof a second sound moderator was established, it would not displace thosescientific findings.
"The existence of Sheila Caffell's blood deep within the sound moderator, incircumstances in which she could not physically have shot herself with themoderator attached, renders irrelevant the putative existence of further sound moderators."
Ms Darlow argued the CCRC had previously "looked extremely extensively at the issue of whether or not there was a second sound moderator" and said Bamber was "tribunal shopping to avoid going back to the CCRC".
This case is the latest legal action brought by Bamber in his long-runningbattle to clear his name.
Mr Justice Julian Knowles reserved his judgment on Bamber's application and said he hopes to give a written ruling next week.