Prime Minister Theresa May has backed calls for the system to be overhauled. 

Justice Secretary David Lidington wants more transparency after the decision to free Worboys was made without all his victims being informed. 

Calls are mounting for the risk assessment into the release to be made public after it emerged his lawyers planned to appeal against his sentence to the European Court of Human Rights. 

He is leaving prison after serving just nine years of an indeterminate sentence. 

Mr Lidington said: “John Worboys was convicted of horrific crimes.  

“My thoughts and unreserved sympathy are with his victims for whom news of the Parole Board’s decision to order Worboys’s release must have reawakened the most appalling memories. 

“It is a priority for this Government that victims of rape and sexual assault have full confidence in the criminal justice system. 

“While sentence lengths for these horrific crimes have increased by more than 30 per cent since 2010, and more victims are coming forward, there is still more to do. 

“While it is right that the Parole Board should remain an independent body, I believe that there is a strong case to review how to allow greater openness about the decision-making process. 

“We also need to make sure arrangements across the criminal justice system ensure victims are both heard and, if they wish, kept informed about their case.”  

Mr Lidington said he had had talks with the chairman of the Parole Board, Nick Hardwick, and Victims’ Commissioner Baroness Newlove about what changes could provide greater transparency and ensure victims are heard and kept informed about their case.

He added: “I want to make sure we consult victims groups and others, and to start this work now so that decisions can be taken before Easter.” 

A Number 10 source said Mrs May fully backed the move, having taken a personal interest in the furore surrounding Worboys’s release, which has been condemned by victims’ campaigners. 

The source said: “Like everyone else, the Prime Minister’s thoughts are with the victims of Worboys’s horrendous crimes and the pain and the suffering they have endured.  

“The Prime Minister welcomes the fact that the whole process is going to be reviewed so we can improve the system for the future. 

“While it is right that parole decisions should be taken by independent boards and not politicians, clearly there has been a lack of transparency in this case and we need to make things a lot better for victims.” 

Worboys, a former stripper and adult film star, was jailed indefinitely in 2009, with a minimum term of eight years after being found guilty of attacks on 12 women over 18 months involving rape, assaults and drugging. 

Police believe he may have targeted more than 100 other victims. 

A pre-sentence report described him as a “repetitive predatory sexual offender”.  

Both victims and police hoped the indeterminate sentence of imprisonment for public protection (IPP) would mean he would be reassessed after eight years and kept behind bars. 

However, a source said: “In 2012 three IPP prisoners applied to the European Court of Human Rights about the legal status of their detention and that court decided that indefinite detention was both disproportionate and unlawful. 

“It has been suggested that Worboys’s lawyers were planning to go down similar channels.” 

A leading campaigner yesterday backed the calls for the risk assessment that led to the release of Worboys to be made public. 

Victims’ Rights Campaign founder Harry Fletcher said: “This is such an extraordinary case that the Parole Board must reveal their detailed reasons for releasing him.  

“And that should include what evidence they have that he addressed his offending behaviour while in prison.” 

Otherwise, he said, he feared Worboys could strike again. 

Mr Fletcher added: “It is extremely difficult to believe that this man does not pose a risk.” 

And he asked: “Why was he only charged with one count of rape? 

“Had he been charged with more than one offence then his prison sentence would have been a lot longer.”  

Mr Fletcher also revealed that taxpayers will have to fork out almost £400,000 over the next decade for his housing and surveillance costs. 

Worboys, 60, will spend the first few years out of prison in a hostel on a dawn-till-dusk curfew before being moved to supported accommodation which will cost £15,000 a year. 

Because Warboys is feared to be such a predatory sexual offender, two teams of 12 police officers will keep a round-the-clock eye on him to make sure he does not reoffend Warboys’s victims had been drinking in bars in London’s West End. 

He pretended to have won the lottery and to celebrate offered them champagne, wine and vodka laced with powerful sedatives.  

Typical was Carrie Symonds, then 19, picked up and given a spiked vodka by Worboys as she made her way home from a night out in Chelsea to East Sheen. 

There is no current police inquiry into Worboys. 

But the Crown Prosecution Service said three rape and sexual assault cases not presented at his trial could now be used against him.