Victims of black cab rapist John Worboys will head to the High Court later today to challenge his release from jail.
Two victims and London Mayor Sadiq Khan were given permission last month to mount a challenge over the lawfulness of the decision to free the serial sex attacker.
Three judges in London will hear the action against the Parole Board over two days and Worboys, 60, is expected to appear via videolink from prison.
He remains behind bars because a temporary bar on his release, initially imposed by the High Court in January, was renewed by Sir Brian Leveson and Mr Justice Garnham during February's hearing.
Worboys was jailed indefinitely in 2009, with a minimum term of eight years, for drugging and sexually assaulting female passengers in his cab.
Many of his victims had been drinking in West End and Chelsea bars and Worboys would offer them champagne spiked with powerful sedatives to celebrate a made-up lottery win.
He would attempt to prove this by showing them a carrier bag stuffed with cash.
He would then pounce on his victims in the back of his vehicle as they were left insensible and unable to protect themselves.
Worboys, who now goes by the name John Radford, was convicted of 19 offences against 12 victims, but has been linked to more than 100 complaints.
A Parole Board decision that he should be freed on licence after spending less than a decade behind bars was widely criticised by his victims and politicians.
Phillippa Kaufmann QC, for the victims, told the court on the previous occasion that it appeared something had gone "very wrong", and they were "concerned not only for themselves but for the wider public and women in particular".
The victims were also concerned, she said, that the release decision was "made entirely in secret".
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The Sun newspaper is also taking part in the legal action after Sir Brian and Mr Justice Garnham gave permission to proprietor News Group Newspapers to challenge the Parole Board's refusal to publish its decision.
The Sun's lawyers say the refusal was "unlawful" and breaches the newspaper's right to freedom of expression.