"Black cab rapist" John Worboys has admitted attacks on four more women.
The 62-year-old taxi driver was jailed in 2009 for a string of sex assaults on 12 victims in London.
He pleaded guilty at the Old Bailey to administering a drug with the intention of carrying out indecent assaults, rape or sexual activity on four women, and will be sentenced on 2 September.
Worboys, from Enfield, targeted women who hailed his cab and drugged them in order to sexually assault them.
The defendant, who now uses the name John Derek Radford, was jailed for at least eight years after his first trial for the attacks carried out between 2006 and 2008.
'Sense of sexual entitlement'
A previous hearing in the latest case was told one victim hailed Worboys' taxi in London's West End in 2000 or 2001.
Prosecutors said the defendant told her he had won money on the horses before offering her champagne laced with drugs. She awoke the next day naked and with no memory of what happened after accepting the drink.
The second complainant was a university student targeted in 2003 after leaving a nightclub on New Oxford Street, in "an identical method to not only the first count, but a number of previous convictions and allegations three and four", the court heard.
Police believe Worboys may have carried out more than 100 rapes and sexual assaults. He was told after his first trial he would be held in custody as long as he was deemed a danger to the public.
Last year, the Parole Board ruled he should remain in prison citing his "sense of sexual entitlement" and desire to control women.
One of his two victims who won a High Court ruling preventing Warboys' release said she was "completely in shock" at his guilty plea.
"Whilst I can't help being cynical about his motives, I am pleased that his victims have been saved the trauma of a court case," she said.
By Danny Shaw, BBC Home Affairs correspondent
John Worboys is staying in prison.
Although the Parole Board had already decided that in November after reconsidering his case on the orders of the High Court, these convictions are almost certain to lead to him serving much longer behind bars; he may even get a life sentence.
That's important for his victims and for public safety.
But his guilty pleas - two of which relate to offences between 2000 and 2003 - are also significant because they prove the former cab driver's crimes stretched further back than he had previously admitted.
Worboys told the parole panel which originally sanctioned his release that his offending started towards the end of 2006, triggered by the breakdown of a relationship.
In the words of Phillippa Kaufmann, the barrister who led the legal challenge, Worboys' account of his criminal behaviour has now been "entirely undermined".
Worboys admitted two counts of administering a stupefying or overpowering drug with intent to commit rape or indecent assault and two of administering a substance with intent to commit a sexual offence.
Gregor McGill, legal director of the Crown Prosecution Service, described Worboys as a "sexual predator".
He said: "Worboys has left countless victims feeling traumatised, anxious and violated. His modus operandi will be familiar to any victim unfortunate enough to cross his path."
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