Stephen Mitchell was ordered to serve two life sentences and warned he might never be freed from prison by a judge at Newcastle Crown Court in January 2011.
Mr Justice Wilkie said the Northumbria Police constable was a "ruthless sexual predator" who was a danger to women.
He ordered that Mitchell should serve at least seven-and-a-half years in jail before being considered for parole, adding: "Thereafter, you will only be released, if at all, if the parole board has concluded it is safe and in the public interest for you to be released on public licence.
"That may not be for many years, if for ever."
Chair of the Commons Home Affairs Committee Yvette Cooper has called for an urgent update from Justice Secretary David Gauke following reports of Mitchell's release on parole.
Ms Cooper said: "This case is disturbing and incomprehensible.
"For someone who has committed such appalling crimes and been told that they may not be released for many years, if ever, to then be subject to release on parole after only seven-and-a-half years is just impossible to understand.
"We urgently need to know what the Parole Board's reasons were because to most people this really does not look like justice for victims."
The Labour MP also pointed to the recent furore which followed the release of another serial sex offender, John Worboys.
Ms Cooper said: "The Justice Secretary also needs to tell us urgently what progress he has made since the decision on John Worboys to stop the same things happening all over again.
"It shouldn't be left to victims to take legal action to get answers about why someone is being released or to stop serious and dangerous criminals being released early."
In a statement, Rape Crisis England and Wales described the case as "deeply disturbing" and said that it highlights "the urgent need for greater clarity around Parole Board decision-making".
Discussing victims' experiences of the criminal justice system, the organisation added: "The criminal justice process takes too long, can be re-traumatising, and sometimes, as this case highlights, even when a dangerous, serial offender is convicted, measures taken to minimise the potential harm they can cause seem severely inadequate."
Mitchell, who was 42 when he was jailed, preyed on women he met while on duty from his base at Pilgrim Street police station in Newcastle.
Originally from Glasgow, he raped and sexually abused heroin addicts, shoplifters and a disabled teenager by offering them help while in custody, then demanding sexual favours afterwards.
He told one of his victims that if she complained, "no-one would believe a junkie".
Sentencing him at Newcastle Crown Court, the judge said: "So cowed and downtrodden by their experiences of life and by your influence were those seven women that they did not report what you had done until they were given the opportunity to do so by the police investigation into youractivities years later."
The former soldier, who resigned from Northumbria Police, was nicknamed Pc Cucumber by investigating officers for his cold, callous demeanour.
After the case, Northumbria Police has apologised for Mitchell's "evil" actions.
The force said serious failings had led to the Pc being free to attack drug addicts and shoplifters after arresting them.