LONDON (Reuters) - London Mayor Sadiq Khan joined a chorus of protest on Friday against a government decision not to block the release from jail of a former black-cab driver suspected of assaulting dozens, possibly hundreds of women.

FILE PHOTO - The Mayor of London, Sadiq Khan, speaks at the Fabian Society New Year Conference, in central London, Britain January 13, 2018. REUTERS/Simon Dawson

Khan said it was “extremely disappointing” that the government was not challenging a decision by the Parole Board to free John Worboys, 60, after nine years in prison.

The opposition Labour Party, of which Khan is a member, also called it deeply regrettable.

Worboys, one of Britain’s most prolific sex offenders, was convicted in 2009 of raping or sexually assaulting 12 women. However, police fear he may have had more than 100 victims.

On Friday, justice minister David Gauke told parliament he had received legal advice that suggested launching a judicial review of the Parole Board decision would not be successful.

Mayor Khan, who has previously said Worboys should not be allowed to set foot in London, said he had instructed lawyers to explore the possibility of a legal challenge from him.

The decision to release Worboys is “astonishing and it is extremely disappointing that the government is accepting this without challenge,” Khan said in a statement.

Worboys’ release has provoked outrage from campaigners against sexual violence and exposed failings in the justice system after some of his victims were not informed of the decision to release him.

Gauke declined to explain his decision further, saying that publishing the legal advice could harm attempts by other groups to challenge his release.

Worboys, a licensed London black-cab driver, used alcohol and drugs to incapacitate his victims between 2002 and 2008.

He told some women he had won money at a casino or lottery and offered them spiked champagne laced with sedatives in an invitation to celebrate with him.

He then pounced on the victims in the back of his black cab and many were left with little memory of their ordeals and could only recall falling asleep in the back of the vehicle before waking up at home.

In 2009, the former male stripper was convicted of 19 offences of drugging and sexually assaulting women and ordered to serve at least eight years in jail. Worboys spent just under 10 years in custody, including a period on remand.

A three-member panel of the Parole Board decided at the end of last year Worboys could be released as he no longer presented a risk to the public.

(Corrects spelling of Worboys’ surname.)