Legal efforts to keep black cab rapist John Worboys behind bars have been launched by the Mayor of London.
Sadiq Khan formally lodged an application on Thursday for a judicial review into the decision to release the serial sex attacker from jail.
The Labour mayor said he was fighting to put the "safety of Londoners and the needs of victims first" after beginning the legal battle at the High Court.
The Parole Board provoked fury this month when it directed Worboys's release after a decade in prison.
He was jailed indefinitely in 2009, with a minimum term of eight years, for drugging and sexually assaulting women passengers.
Despite being convicted of 19 offences against 12 women, it is feared Worboys may have more than 100 victims.
Mr Khan's decision to begin a legal challenge came after Justice Secretary and Lord Chancellor David Gauke ruled out a judicial review.
The mayor said: "I have today lodged with the Court an application for judicial review of the Parole Board's decision to release the convicted rapist John Worboys.
"It was an astonishing and deeply concerning decision that simply cannot go unchallenged. For victims, and all Londoners, it must be properly scrutinised in the courts. The Chair of the Parole Board has welcomed this scrutiny.
"I will always put the safety of Londoners, and the needs of victims, first - which is why I instructed leading Counsel to begin this process last week.
"Londoners need to know that those in authority are doing everything they can to keep them safe, and that means keeping dangerous individuals off our streets."
Notorious predator Worboys has already been moved to a London jail ahead of his release, prompting outrage among his victims based in the capital.
The 60-year-old was transferred from HMP Wakefield in West Yorkshire to HMP Belmarsh, south-east London, at the weekend, it is understood.
On Tuesday, police said they were investigating a fresh allegation of historical sexual assault which is understood to have been made against Worboys.
The incident was said to have taken place in 1997 and was reported to police this month. Worboys has not been arrested.