Mayor of London Sadiq Khan has launched a legal effort to block the release of black cab rapist John Worboys.
The former Labour MP has lodged an application for a judicial review of the Parole Board's decision to release Worboys.
Earlier in January, the Parole Board sparked fury when it announced Worboys would soon be eligible for release after 10 years in prison.
He was jailed indefinitely in 2009, with a minimum term of eight years, for drugging and sexually assaulting female passengers.
Despite being convicted of 19 offences against 12 women, it is feared the 60-year-old could have more than 100 victims.
On Tuesday, police revealed they were investigating a fresh allegation of historical sexual assault against the former stripper, which is said to have taken place in 1997 and was reported to police earlier in January. Worboys has not been arrested in connection with the claim.
A judicial review would see a judge review the lawfulness of a decision made by a public body - in the case of Worboys, the Parole Board.
It is less about whether the decision was right or wrong, and more about challenging whether proper procedures were followed to reach the decision.
Mr Khan said he was launching his legal battle at the High Court because he believes the decision to release Worboys "was astonishing and deeply concerning" and "simply cannot go unchallenged".
The mayor's move comes after Justice Secretary and Lord Chancellor David Gauke ruled out a judicial review.
He continued he had lodged the application in a bid to put the "safety of Londoners and the needs of victims first".
Also on Thursday, two of Worboys' victims lodged a separate application for a judicial review into the Parole Board's decision and includes a request for urgent consideration seeking a stay in the release of the rapist.
The two women, who say they were not informed of the decision to release Worboys, say their application for a judicial review is based on the claim that the decision not to disclose the Parole Board's decision to the public is unlawful, as it flies in the face of open justice.
Their second argument is that the Parole Board's release decision was "irrational" given Worboys' "long history of cold calculated attacks on a large number of women".
A solicitor for the pair said they "consider they must do all they can to reduce the risk to other women and seek public accountability".
Lawyers also intend to present police and prosecutors with a dossier of allegations against Worboys, saying they were contacted by a "number of other victims".
Lawyer Harriet Wistrich said the raft of evidence she intends to hand to detectives "includes two previously unreported cases from 2002 and 2003 and two cases that were investigated by the police that would pass the evidential threshold but have not been prosecuted".
It is understood that Worboys has already been moved to a London prison ahead of his release, sparking outrage among his victims based in the capital.
The notorious predator is said to have been tranferred from HMP Wakefield in West Yorkshire to HMP Belmarsh, at the weekend.