Victims of John Worboys are preparing a legal challenge over the decision to order his release from prison.
Lawyers for two women are poised to seek a judicial review of the Parole Board's direction that the notorious sex offender be freed after a decade in prison.
It comes as ministers weigh up whether to pursue their own challenge in the courts after the decision sparked outrage.
Worboys, a former black taxi driver, was jailed indefinitely in 2009, with a minimum term of eight years, for drugging and sexually assaulting women passengers.
The 60-year-old was convicted of 19 offences against 12 victims but has been linked to more than 100 complaints in total.
Harriet Wistrich, solicitor at Birnberg Peirce and director of the Centre for Women's Justice, said: "This will be an unprecedented legal challenge.
"There is an arguable basis in a case raising such concern to challenge the Parole Board rules preventing publication of reasons for its decision.
"On the basis of all the known facts in this case, the decision to release Worboys appears to be so irrational.
"If we get access to the reasons then we can explore grounds for challenging a decision which has caused so much alarm and is horrific for all the victims concerned."
A crowdfund page has been set up to support the two women, who cannot be identified for legal reasons, in their challenge.
One said: "I can't watch the news or read the papers. My heart freezes when I hear his name. Seeing his face makes me feel unwell. He's ruining my life all over again."
A "letter before claim" has been sent to the Parole Board requesting that the release of Worboys be halted pending the outcome of the legal proceedings.
Lawyers also flagged up a previous decision to deny Worboys a move from a high-security prison to an "open" establishment two years before he was found to be safe to be freed into the community.
In September 2015, the board determined against transferring him to open conditions from a Category A jail.
That decision was made on the basis of reports and documents but no oral hearing was held.
At the weekend it emerged Justice Secretary David Gauke was looking into the possibility of taking the highly unusual step of seeking a judicial review.
He has commissioned advice on the possible action, with a decision expected in the coming days.
On Tuesday Nick Hardwick, the chairman of the Parole Board, delivered a staunch defence of the organisation, warning against "political interference" in its decisions.
He acknowledged that the board should be open to scrutiny and legal challenge, saying he would welcome a decision by the Government to seek a judicial review.
Professor Hardwick said: "I hope such a review will provide assurance that the Parole Board itself has acted in accordance with the law and the evidence."
He said he did not think it is right that the board is barred from disclosing details of individual cases and welcomed a Government review to examine measures to boost transparency.