Victims of black cab rapist John Worboys begin a High Court challenge against what they describe as the irrational decision to release him from jail.
Three judges in London will also hear an action against the Parole Board by London Mayor Sadiq Khan.
Two victims and Mr Khan were given the go-ahead at a High Court hearing last month to mount full challenges over the lawfulness of the decision to free the serial sex attacker.
At the hearing in February, 60-year-old Worboys listened to proceedings from the dock of the courtroom after being transported from prison.
Worboys is expected to appear via video link from prison for the latest hearing.
Sir Brian Leveson and Mr Justice Garnham, who allowed judicial review proceedings to be pursued after hearing argument last month, also ordered that a temporary bar on Worboys' release, initially imposed in January by the High Court, should be continued.
Sir Brian and Mr Justice Garnham, along with a third judge, Mr Justice Jay, will now hear submissions over two days on behalf of all the parties involved.
Worboys was jailed indefinitely in 2009, with a minimum term of eight years, for drugging and sexually assaulting women passengers.
He became known as the black cab rapist after attacking victims in his hackney carriage.