Black-cab rapist John Worboys could be freed within weeks after the Government decided not to challenge his release.
Justice Secretary David Gauke said it would "not be appropriate" to seek a judicial review of the case after taking "considered and expert" legal advice.
He warned the bar would have been "very high" and that there was "no reasonable prospect of success".
Speaking in the House of Commons, Mr Gauke told MPs: "I believe that I owe it to those victims and to the public to consider all the options open to me.
"I therefore took the step of seeking legal advice from specialist leading counsel to establish whether there were grounds to challenge this decision in the courts and therefore to ask the court to stop the release of Worboys before the decision was reconsidered."
He added: "Having taken considered and expert legal advice I have decided it would not be appropriate for me as Secretary of State to proceed with such a case.
"Honourable members will appreciate that I cannot go further and expose detail of the legal advice I have been given."
Mr Gauke did suggest he supported victims seeking their own advice, saying: "Just because I am not taking action, does not mean others cannot."
Two women raped by Worboys have launched a crowdfunding appeal to raise money to challenge the decision themselves.
Labour MP Jo Stevens said that was because legal aid in judicial review cases had been "severely restricted" by the previous coalition government – and should be looked at again in light of the Worboys decision.
Mr Gauke responded that he would not make a decision until after a review into the Parole Board had finished – expected by Easter.
He did reveal the review would be widened to look at the way victims are contacted about their attacker's release.
The Parole Board was forced to apologise and accused of leaving victims "in the dark" after terrified women attacked by Worboys complained they had not been told about the decision to release him.
The Board said widening the review was a "sensible course of action" to make sure the public and victims have confidence in how it works.
Mayor of London Sadiq Khan also announced he would be seeking a judicial review.
Worboys has served a year more than the minimum eight-year sentence he was given for one rape, six sex attacks and the drugging of 12 women.
Police believe he attacked at least 100 victims, but more allegations were not pursued either because evidence was poor, or due to the fact that more convictions would not have added to his sentence, prosecutors said.