Black cab rapist John Worboys is facing life in jail after admitting to four more attacks on women.
The 62-year-old, who now goes by the name John Derek Radford, targeted women who hailed his cab and drugged them in order to sexually assault them.
In 2009, he was jailed indefinitely for the public protection with a minimum of eight years for attacks on 12 women.
At an Old Bailey hearing on Thursday, the defendant, who appeared via video-link from Wakefield Prison, pleaded guilty to further offences against four more victims who came forward last year.
He pleaded guilty to two counts of administering a stupefying or overpowering drug with intent to commit rape or indecent assault.
He admitted two further charges of administering a substance with intent to commit a sexual offence under the Sexual Offences Act.
Adjourning the case to September 2, Justice Maura McGowan said the maximum sentence Worboys faces is life in prison.
Ordering a report on Worboys' history of crime, she said: "This is a case on which the public interest is better represented by the Probation Service putting forward a complete history."
At his first trial at Croydon Crown Court in 2009, jurors were told that Worboys picked up his victims in London's West End and plied them with champagne laced with sedatives on the pretext of celebrating a lottery or casino win.
Worboys was convicted of 19 offences including one count of rape, five sexual assaults, one attempted assault and 12 drugging charges.
Last year, the Parole Board ruled that he should remain in prison, citing his "sense of sexual entitlement" and a need to control women.
The four women made their allegations to police in early 2018, but the offences dated back to between 2000 and 2008 in London.
Gregor McGill, Crown Prosecution Service director of legal services, said: "John Worboys is a sexual predator. His modus operandi will be familiar to any victim unfortunate enough to cross his path: entice them into his cab with cheap fares, offer them a sedative-laced drink and take advantage of their vulnerable state.
"Worboys has left countless victims feeling traumatised, anxious and violated. I'd like to pay tribute to their bravery in continuing to speak up."