Lawyers acting on behalf of Jeremy Bamber, serving a whole life sentence for one of Britain’s most notorious multiple murders, have sent the Crown Prosecution Service a report by a senior forensics scientist which they claim undermines vital evidence heard at his trial.

The confidential report, which has been seen by the Guardian, casts doubt on the validity of evidence relating to a rifle sound moderator, or silencer, that was pivotal to Bamber’s conviction in 1986.

The jury at Bamber’s trial were told that a silencer had been attached to a rifle used to kill Bamber’s adoptive parents, Nevill and June Bamber, his adoptive sister Sheila Caffell, and her six-year-old twin boys, Daniel and Nicholas.

In the early hours of 7 August 1985, police were called to the Bamber family home, White House farm, near the village of Tolleshunt D’Arcy, Essex. Jeremy Bamber, who lived nearby, had phoned the police saying he had received a call from Nevill Bamber, saying Sheila had “gone berserk” and had one of his guns.

Facebook Twitter Pinterest Jeremy Bamber’s adoptive mother June Bamber, his sister Sheila Caffell and her sons Nicholas and Daniel. Photograph: Trinity Mirror / Mirrorpix / Ala/Alamy

Bamber drove to the farm and met three police officers outside the farmhouse and a firearms team were called to the scene. At 7.35am, armed officers entered the farmhouse. They found Nevill Bamber in the kitchen. His wife, June, Caffell and her twin boys were found upstairs. All had died from gunshot wounds.

Initially, the police and the coroner believed Caffell, a model known as Bambi who had recently been diagnosed with schizophrenia, had shot and killed her family and then turned the weapon on herself. The rifle used was found next to her body. It did not have a silencer attached.

But a silencer was found in the gun cupboard in the house. That would later form a key part of the case against Bamber, who was charged with the five murders and appeared for trial at Chelmsford crown court in October 1986.

The prosecution argued that, motivated by the prospect of inheriting the £436,000 family fortune and considerable land, he had killed all five then placed the rifle in his sister’s hands to make it look like a murder-suicide. He has always maintained his innocence, and has lost two appeals against his conviction.

Jeremy Bamber still waits for the evidence that might clear his name | Simon Hattenstone and Eric Allison Read more

At the trial, it was accepted that there were only two possible killers: Bamber or Caffell. The silencer was a key exhibit. The jury were told it had a speck of blood on it that had come from Sheila. They were also told that, with the silencer attached to the rifle, the additional length meant Caffell could not have shot herself, and that she clearly could not have shot herself then placed the silencer in the gun cupboard.

They were also told that red paint marks on the silencer matched scratch marks on the kitchen wall of the farmhouse where Nevill Bamber’s body was found. The prosecution claimed this showed the silencer had been attached to the murder weapon during a struggle between Nevill and his assailant.

The issue of the silencer was vital in persuading the jury, with the judge instructing them the silencer “could, on its own, lead them to believe that Bamber was guilty”.

A week before Bamber’s trial started, the head of biology at Huntingdon Science Laboratories wrote a letter to Essex Police, seen by the Guardian, saying that the results of the blood tests would show that the blood “could have come from either Sheila Caffell or Robert Boutflour”.

Boutflour, now dead, was a relative of Nevill and June Bamber and gave evidence for the prosecution. He was a regular visitor to White House farm and had used the guns kept there for shooting. Despite the earlier statement, a forensic scientist who had examined the results for Huntingdon Science Laboratories told the jury that only Sheila Caffell’s blood was found in the silencer.

Facebook Twitter Pinterest White House farm at Tolleshunt D’Arcy in Essex. Photograph: Matthew Fearn/PA Archive/Press Association Ima

After the jury were sent to reach a verdict, they returned and asked for clarification regarding the silencer. The judge told them it contained only the blood of Sheila Caffell. Seventeen minutes later, they returned and convicted Bamber by a 10 to two majority. He was sentenced to life imprisonment with an order to serve a minimum of 25 years. This was increased to a whole-life tariff in 1988 by the then home secretary, Douglas Hurd.

Bamber has claimed for many years that evidence was not disclosed to the defence by Essex police showing that two silencers had been examined by forensic scientists.

In the new report, the senior forensic scientist and leading firearms expert concluded that separate silencers were being examined by the police at the same time in different departments, based on “serious discrepancies” in how the devices were described and the fact that there was no record of any item being transferred from one section to the other.

Both silencers are believed to contain blood that could belong to either Caffell or Boutflour. However, Bamber and those supporting him believe the issue of the silencers is a red herring – that Caffell killed the five family members, but no silencer was used.

Facebook Twitter Pinterest Jeremy Bamber and girlfriend Julie Mugford at the funeral of three members of his family a year before he was convicted of their murders. Photograph: PA

In May this year, Bamber’s lawyer Mark Newby received a letter from Frank Ferguson, head of special crimes at the CPS. He had written to the prosecutors’ office on the issue of two silencers being examined in the case but the jury only being told about one.

Although Ferguson stated “there is no documentary evidence either provided or referred to which supports the existence of a second silencer”, he added: “Any evidence that suggests that there was or may have been another silencer for the rifle would raise the possibility that the other silencer was used during the shooting and not the one alleged by the prosecution.

Jeremy Bamber talks about fresh appeal against murder conviction - audio Read more

“Such a possibility would significantly undermine the case against JB [Jeremy Bamber] and any material supporting such a possibility would plainly be material which casts doubt on the safety of the conviction.”

Now Bamber believes he has the evidence to undermine the case against him. Writing to the Guardian from Wakefield prison, Bamber said: “The report in the hands of the CPS proves with absolute certainty that this case featured two silencers. And now the CPS must act. There are moments in life when the truth can no longer be suppressed and this is one of those times.”

Newby, a solicitor advocate from Quality Solicitors Jordans, said: “I hope this report will persuade the CPS to now authorise the handing over of all the undisclosed material still held by Essex police in this case.”

A spokesperson for the CPS said: “We have received correspondence relating to this case and requested additional material in order to respond to the points raised.”

An Essex Police spokesperson said: “Essex Police has no comment to make on these claims given that Jeremy Bamber’s conviction has been the subject of several appeals and reviews by the Criminal Cases Review Commission and there has never been anything to suggest that he was wrongly convicted.”

• One of the authors of this article, Eric Allison, is a patron of the Jeremy Bamber campaign.