The chairperson of the Parole Board has warned against “political interference” in its decisions, following an outcry over the impending release of black cab rapist John Worboys.

Nick Hardwick delivered a staunch defence of the organisation after it faced intense criticism for directing one of Britain’s most notorious sex offenders be freed after a decade behind bars, the Press Association reported.

It has emerged Worboys was denied a move to an open prison two years before he was assessed as safe to be released.

In September 2015, the board decided against transferring him to open conditions from a high security jail.

That decision was made at the “paper stage” – meaning reports and documents were assessed but no oral hearing held.

Professor Hardwick acknowledged the board should be open to scrutiny and legal challenge, saying he would welcome a move by Justice Secretary David Gauke to seek a judicial review of the Worboys’ decision.

He added: “I hope when people think about it, they will agree it is right we resist political interference in our decisions. Like any court, the Parole Board members must make independent decisions in accordance with the law and on the basis of evidence.

“It would be a bad day for us all if people’s rightful abhorrence of Worboys’ crimes or even justified concern about a Parole Board decision allowed these basic principles of justice to be overturned.”

An outcry erupted earlier this month after it emerged Worboys, a 60-year-old former black taxi driver, was to be released from prison.

He was jailed indefinitely in 2009, with a minimum term of eight years, for drugging and sexually assaulting women passengers.

He was convicted of 19 offences relating to 12 victims but has been linked to more than 100 complaints in total.

At the weekend, ministers pledged to do everything possible to ensure Worboys stayed behind bars. It emerged the justice minister was looking into the possibility of taking the highly unusual step of seeking a judicial review.

In a detailed statement on Tuesday, Hardwick noted public concern about the case was “completely understandable”.

He described how for prisoners such as Worboys, once they have served the minimum part of their sentence, the board must determine whether they are safe to release.

“The law governing the Parole Board’s decisions is quite clear.  We have to make decisions about future risk,” he said.

“We cannot re-assess the prisoner’s guilt or innocence or whether the original sentence was appropriate even if we would like to do so.

“The decision about future risk will be informed both by evidence of how the prisoner has changed and the robustness of plans to manage him or her in the community.”

Hardwick stressed risk could not be “eliminated completely”.

He said: “Parole Board members need to be confident a prisoner will not reoffend – but they cannot be certain. If certainty is required, that needs to be reflected in the length of the original sentence.”

The law bars the board from revealing the full reasons for its decisions, however Hardwick gave fresh details of the processes that were followed in the Worboys case.

A hearing was held last year by a three-person panel chaired by one of the board’s most experienced female members.

The panel considered a dossier of 363 pages and heard evidence from psychologists and prison and probation staff responsible for Worboys.

It was also revealed the government was represented at the hearing at the board’s request, while Worboys himself was questioned “in detail”, and the panel considered a written statement from one victim.

Anger over the case intensified amid complaints victims were not kept informed over the case or the licence conditions that will be placed on Worboys.

Hardwick said he shared the concerns but noted the Parole Board had no role in contacting or liaising with victims.

He said: “That is undertaken by the Victim Contact Service, part of the National Probation Service, on behalf of the Secretary of State.”