Dominic Raab has been accused of a “catastrophic” decision that experts say profoundly undermines public safety by allowing prisoners to abscond and others to commit serious offences while on parole.

In a strongly worded letter to the justice secretary, three unions castigate a “momentous and dangerous” move by Raab to ban psychologists, prison staff and probation officers from informing the Parole Board whether they believe prisoners should be released.

Under the previous system, parole panels considering applications for release were given reports including risk assessments and recommendations from professionals who had worked with the inmate. On Thursday, however, the system changed so that the parole panel will receive the reports without recommendations.

The letter, from the GMB/Scoop, Unison and the probation officers’ union, Napo, said that eradicating “crucial expert opinion in the arena of public protection” presented “clear dangers” to the public. It urged Raab to overturn the rule change, which the writers say was made without consultation and approved by parliament without debate on 30 June. The new rules affect all staff working for the prison and probation service, including forensic psychologists who have assessed the inmate, prison officers who have worked with them, and probation officers who have monitored them.

The unions’ letter to Raab, dated 19 July, says: “[This] severely endangers the ability of the probation service to protect victims of the most serious offences, and indeed the wider public, from the risk of serious harm posed by many individuals involved in the parole system.” The justice secretary attempted to explain the reason for the rule change in a parliamentary exchange on 5 July when the Labour MP Kate Green asked why experts would be “forbidden to provide a view on suitability for release of the most serious offenders”.

Raab said: “At the moment, when the vital question of risk is assessed, there is a risk that separate reports, whether from psychiatrists or probation officers and those who manage risk, may give conflicting recommendations. Therefore, in those serious cases … there will be one overarching Ministry of Justice view, so that the Parole Board has a very clear steer.”

In essence, it means that Raab, in a few serious cases, will offer a “single view”, but in most cases, the Parole Board will be left to reach a decision without any recommendations.

The letter also says this “ill-conceived and haphazardly executed” development would undermine confidence in the criminal justice system, and increase costs and the workload of agencies.

It also warns that Raab’s rule change risks him being remembered in a similar way to one of his predecessors, Chris Grayling, who is still widely mocked for his discredited attempts to revamp the probation service.

A Ministry of Justice spokesperson said: “Protecting the public is our top priority and prison and probation staff will continue to provide the same detailed evidence to inform the Parole Board’s decisions.

“Our reforms will put more people with law enforcement experience on Parole Board panels and add a ministerial check on release of the most serious offenders – keeping our streets safe.”