Victims of rapist John Worboys were given written assurances about how long he would spend in jail after he was convicted, it has emerged.
The CPS told victims the indeterminate term he received was "to all intents and purposes" a life sentence.
Letters featuring the details, sent to his victims, have been seen by the BBC.
Worboys, 60, was jailed for a minimum term of eight years in 2009 for drugging and sexually assaulting women passengers.
News that the Parole Board had decided to free Worboys after a decade behind bars was met with a furore by his victims and in Parliament.
After Worboys' trial in 2009, the special casework lawyer for the CPS, Anthony Connell, wrote to victims to explain the sentencing process.
He said: "An indeterminate sentence is a new sentence that has been introduced only a few years ago.
"To all intents and purposes, it is a sentence of life imprisonment."
Mr Connell said it meant Worboys could apply for release after serving his minimum term.
"Merely because he is allowed to apply after eight years, does not mean that he will be granted parole," he wrote.
"It is clear from the judge's sentencing remarks that he is considered a dangerous offender.
"Those responsible for considering any application by him will no doubt have in mind this fact together with the fact that he committed an awful lot of serious offences."
Mr Connell, who retired in 2015, concluded his letter by saying he hoped the conviction and sentence had brought the victims "some peace of mind".
Five days after Worboys' trial ended, Det Insp David Reid wrote to the victims and confirmed 85 women had come forward to police.
He also set out a series of questions victims wanted answering, including: "Is he [Worboys] likely to be charged with other offences now?"
Det Insp Reid replied: "No. This is partly because of the lengthy sentence anticipated. Also, given the significant publicity that the case has attracted, it is unlikely that he would be able to obtain a further fair trial."
The Scotland Yard detective went on to say that Worboys was a "unique offender" who was "dangerous" and had "exacerbated the distress by refusing to accept what he has done".
At his trial, jurors heard Worboys picked up his victims in London's West End and gave them champagne laced with sedatives, claiming he had won the lottery or had won money at casinos.
He was convicted of one rape, five sexual assaults, one attempted assault and 12 drugging charges.
As well as being ordered to serve at least eight years, Worboys was given an indeterminate sentence, meaning he could be kept in prison as for as long as he was deemed to remain a danger to the public.