Lawyers representing Jeremy Bamber, who is serving a whole life sentence for killing five members of his family in 1985, will ask the high court to review the refusal of the Crown Prosecution Service to disclose evidence they believe could undermine his conviction.

They claim the material was made available to an author working on a televised drama about the killings, but not to his defence team.

Earlier this year, Carol Ann Lee, the author of the book on which the recent ITV docudrama, White House Farm, was based, said Essex police had assisted her research by providing her with documents from the case.

Lee posted an image on Instagram showing a series of numbered documents that have not been disclosed to the defence. She wrote on Instagram: “It was interesting to see and read all the original material.”

The numbered documents taken from Holmes boxes (an administrative system introduced in 1985 to support major investigations) include files numbered 75/13 and 75/15, which Bamber says he has never seen. After Bamber’s legal team complained to the CPS the Instagram post was deleted.

Bamber, aged 59, has spent the past 35 years sifting through millions of documents in an attempt to prove his conviction is unsafe. Because the files in the Holmes boxes are all numbered, he believes certain files have not been disclosed.

The hearing on Friday in Leeds follows the refusal by a high court judge to judicially review the decision of the CPS not to disclose material to Bamber’s defence team. That was a “paper hearing” in which the judge considered the written evidence in private without representation from Bamber’s legal team. At Friday’s hearing, known as an oral renewal, the case will be fully reconsidered, supported by oral submissions from Bamber’s legal team.

Bamber’s adoptive parents Nevill and June Bamber were shot and killed inside their Essex farmhouse during the night of 6-7 August 1985, along with their adopted daughter, Sheila Caffell, and her six-year-old twin boys. Bamber, then 24, had phoned the police to say Nevill had phoned him, saying his sister, Caffell, had “gone crazy and has the gun”.

Initially, police believed that Caffell had fired the shots then turned the gun on herself. But, on 10 August, after the police ended their examination of the crime scene, a relative of Nevill and June Bamber, David Boutflour, found a silencer in the gun cupboard of the farmhouse. It was later said to contain blood belonging to Sheila Caffell.

On 7 September 1985, Jeremy Bamber’s ex-girlfriend told police Bamber had discussed killing his family with her and that he was involved. On 29 September 1985 Bamber was charged with the murders.

After the jury was sent out to reach a verdict, it returned and asked the judge for clarification on the silencer and blood evidence. The judge said it contained only the blood of Sheila Caffell. Seventeen minutes later, the jury returned and convicted Bamber by a 10 to two majority.

In 2018 the Guardian revealed that a week before the trial, the head of biology at Huntingdon Forensic Science Laboratories wrote to Essex police saying the blood on the silencer “could have come from Sheila Caffell or Robert Boutflour”, another relative. That letter was not disclosed to the defence.

The Guardian also revealed that a peer-reviewed report compiled by Philip Boyce, a ballistics expert at Forensic Equity Ltd, was sent to the CPS in 2019 about the possibility that there had been more than one silencer. The CPS has dismissed his findings, without employing its own expert to study his report.

At the trial the prosecution argued that Caffell could not have killed herself because she had been shot in the neck twice. Earlier this year, the Guardian revealed statements made by senior police officers who attended the crime scene had been uncovered, stating that they had only seen one bullethole in Caffell’s neck. Again, these statements were not disclosed to the defence. Bamber believes other vital records have not been disclosed.

Mark Newby a solicitor advocate at Quality Solicitors Jordans, which represents Bamber, said: “Since proceedings were issued it came to our attention that the author of the book the ITV drama was based upon appears to have received material directly from the police.

“It cannot be right that an author has been given material that Mr Bamber’s defence team have not seen, particularly in light of the persistent refusal by the CPS to disclose specific material we have been asking for. It raises the question of whether a work of fiction is more important than justice.”

Bamber, speaking from Wakefield prison, told the Guardian: “It is disgusting that a third party was provided with documents and sensitive photographs when Essex police and the CPS have been unwilling to hand over material to our own forensic experts to prove that two silencers were recovered from the scene.

“Despite court orders being in place for them to make full disclosure to us, which they deliberately chose to ignore, when Carol Ann Lee requested material she was given it without a second thought. Where is my justice?”

A spokesperson for Essex police said: “We are aware of the claim made by Jeremy Bamber and can confirm that this matter forms part of an ongoing judicial review. As this matter is therefore the subject of a forthcoming legal hearing it would not be appropriate for us to comment further.”

On 1 May, the case was adjourned to 29 May.

• This article was amended on 1 May 2020. The name of the ITV docu-drama was White House Farm, not White House Farm Murders. This has been corrected.