Jeremy Bamber, who has served more than 40 years in prison for murdering five members of his family, has been banned from communicating with the media.

Bamber was convicted in 1986 by a 10-2 majority of shooting his adoptive mother and father, his sister and her six-year-old twins at the parents’ family farmhouse in Essex a year earlier. He has always protested his innocence.

The 65-year-old has long relied on telephone interviews and exchanges of letters with journalists as a way to draw attention to his case. But his campaign group has told the Guardian he is no longer allowed to send letters to journalists, nor receive them, nor is he allowed to talk to reporters by phone. He was last granted a face-to-face interview with a journalist in 2010.

Without giving a specific explanation for the decision in Bamber’s case, the prison service cited “the need to protect victims from serious distress and maintain confidence in the justice system” as the basis for such restrictions in general.

The ban comes at a time when the case has received prominent media coverage. Last October, the New Yorker magazine released Blood Relatives, a six-part podcast series questioning the safety of the convictions.

Initial newspaper reports of the massacre called it a murder-suicide, stating that his sister, Sheila Caffell, who was also adopted and had recently been hospitalised with schizophrenia, had killed her family members and then herself.

Bamber became a suspect a month later after his former girlfriend Julie Mugford gave a statement to Essex police soon after he had ended their relationship stating he had told her he was planning the murders. It emerged later that Mugford had agreed to sell her story to the News of the World for £25,000 if Bamber was convicted.

This February, Prof Jason Payne-James, a specialist in forensic and legal medicine, told the Guardian he did not believe a silencer had been used in the shootings, a finding that appears to undermine a central tenet of the prosecution’s case. At the trial, Justice Drake told the jury that if they believed the silencer was used it would be sufficient ground to convict Bamber because Caffell’s arms were not long enough to shoot herself in the throat with the extension added to the rifle.

His campaign group told the Guardian that Bamber, who is in the high-security category A prison HMP Wakefield, believes the ban on him communicating with the media is unlawful.

The right of prisoners claiming a miscarriage of justice to contact the media, including by telephone and interview, was established in the UK through a 1999 case, in which Ian Simms and Michael O’Brien – both convicted of murder – successfully argued that a “blanket ban” on journalists interviewing prisoners violated the prisoners’ rights to free speech and obstructed access to justice.

Four years earlier, Bamber had challenged his restricted access to media, after being banned from making a phone call to a journalist, in the European court of human rights. Although his application was ruled inadmissible, the ECHR said this was because he had the right to communicate with journalists in writing. This ruling was superseded by the Simms and O’Brien judgment.

A spokesperson for the Jeremy Bamber Innocence Campaign said: “The ban that the prison has imposed is a sign the authorities are determined to do anything to prevent Jeremy exposing the misconduct, and mistakes that led to his wrongful conviction.

“Wakefield prison has taken this dramatic step in direct contravention of the Ministry of Justice rules, which allow prisoners contact with the media ‘when making serious representations about their own conviction’. It seems to be no coincidence that just as the mass of new exculpatory evidence in his case is receiving a lot of media attention, the prison decides that he can no longer make representations to the media.”

Bamber’s legal team has indicated it will take the prison to judicial review if the ban is not lifted.

Matt Foot, the co-director of Appeal, a charity dedicated to challenging potential wrongful convictions, said: “To deny Mr Bamber the ability to correspond with the media appears to be a blanket ban. Mr Bamber has always claimed his innocence, and I would hope the prison reverses this decision as soon as possible.”

A Prison Service spokesperson said: “Prisoners’ access to the media policy does not provide blanket bans. Any restriction on communication requires justification and will take into account factors such as the need to protect victims from serious distress and maintain confidence in the justice system.”