The government is doing "everything we can" to make sure rapist John Worboys stays in prison, Conservative party chairman Brandon Lewis has said.
Mr Lewis said it was crucial victims were "properly protected", while a lawyer for victims said releasing Worboys would "put many women at risk".
Labour's London Mayor Sadiq Khan said in a tweet that he was "astonished" by its decision, saying "it must listen to Worboys's victims".
"He should not be allowed to set foot in London," he added.
Parole Board chairman Nick Hardwick has said hearing the decision must have been "horrible" for the women, but the board was "confident" Worboys would not reoffend.
He said the fact some victims were not informed was a fault with the parole system, but the decision itself would have been carefully considered.
Worboys, now aged 60, gave his victims drug-laced champagne before attacking them in the back of his cab.
In 2009, he was convicted at Croydon Crown Court of 19 offences and ordered to serve at least eight years in jail.
The following year police said other women had come forward and that the suspected number of his victims now totalled more than 100.
The cabbie offered them Champagne spiked with powerful sedatives to celebrate a fictional lottery win, backed up with a carrier bag stuffed full of cash, claiming he had won the lottery or had won money at casinos.
The drugs left the women insensible and unable to protect themselves as he pounced on them in the back of the vehicle.
In 2014 Mrs Wistrich’s clients won their High Court bid to receive compensation from the Met Police over the way the case was handled.
At the time Mr Justice Green said “systemic” failures in the investigation meant Worboys was not stopped earlier.
Worboys’ period on licence lasts for at least 10 years and he can be sent back to jail if he breaches the conditions.
He has been released after a case hearing in November in which the Parole Board agreed to allow him to go free with ‘stringent’ conditions he must report to probation staff on a weekly basis.
He is also banned from contacting ANY of his victims.
Furious Women's Equality UK said: "The criminal justice system continues to fail to protect women.
Worboys, a former stripper from Rotherhithe, south-east London, has spent 10 years in custody including a period on remand.
Kathy Martin, 44, who had been his girlfriend for a year at the time of his arrest, said she had no idea of his ‘double life’. She said he gave no clues about his predatory night-time activities and she was left dumbstruck when police knocked on her door.
Yvonne Traynor, CEO of Rape Crisis South London, described the time Worboys spent in prison as "woefully short".
She said: "It feels far too soon for this dangerous and manipulative perpetrator to be released into the public having served this woefully short time in prison.
"He may be free to live amongst us but the women he raped will never be totally free from his despicable actions."
Karen Ingala Smith, chief executive of nia, a non-profit organisation that provides services for women, children and young people who have experienced male violence, said: "The police believe that Worboys may have raped more than 100 women, others think this is a conservative estimate.
"He’s served nine years and nine months, that’s just over a month per rape victim. How can we say justice has been served?
"The rehabilitation of offenders is an important principle, but the law should function to protect liberties and rights of the population and I struggle to think anything other than women will be less safe with Worboys on the streets."
Fay Maxted, chief executive of The Survivors Trust, called Worboys' crimes "calculated" and "deliberate".
She said: "Naturally the victims are going to feel, I think cheated that he is being released. You think you're going to be safe when you're in a taxi, so it will be very frightening for his victims to know that he is being released."
Many of Worboys' victims were young women who had been drinking in trendy night spots in the West End and Chelsea.