Two victims of the rapist John Worboys are due to begin a High Court challenge against the decision to release him from prison.
The Parole Board approved his release with “stringent” licence conditions but that decision was widely criticised.
Victims say they were not informed of the decision before it was made public and that women’s safety is at risk.
Worboys, 60, was jailed indefinitely in 2009 for assaults on 12 women in London.
He is believed to have carried out more than 100 rapes and sexual assaults on women in the capital between 2002 and 2008.
London Mayor Sadiq Khan has also launched a judicial review against the action to free the former black-cab driver.
In January, the government said it would not challenge the Parole Board’s decision, saying it was not “appropriate” for the government to seek a judicial review.
During the High Court challenge, three judges will hear evidence over two days from the victims and Mr Khan.
Worboys is expected to appear at the hearing via video link.
At a hearing in February, Sir Brian Leveson and Mr Justice Garnham granted the appeal to challenge the decision.
Sir Brian said Worboys should be granted “emergency legal aid” and ordered the temporary bar on his release to continue.
Lawyer Phillippa Kaufmann, representing the two victims, said it appeared something had gone “very wrong” and the victims were “concerned not only for themselves but for the wider public and women in particular”.