An expert witness testified on Monday that British banker Rurik Jutting, charged with the murders of two Indonesian sex workers, was a sadistic narcissist whose condition was exacerbated by cocaine and alcohol.

Richard Latham, a consulting psychiatrist, told the court Jutting suffered from at least four mental disorders.

Latham, who was called as a defense witness, said the mental disorders experienced by the former Bank of America Merrill Lynch employee did not meet the legal standard for mental illness but "at the moment of killing, the disorders combined to substantially impair his ability to control his behaviour".

Latham said Jutting claimed to have been sexually abused by another boy while at the UK's exclusive Winchester College.

Jutting is accused of murdering Sumarti Ningsih, 23, and Seneng Mujiasih, 26, who were found dead in Jutting's luxury fl at on Nov 1, 2014.

The 31-year-old Cambridge graduate has pleaded not guilty to murder but attempted to enter a guilty plea to manslaughter on the grounds of diminished responsibility. The prosecution rejected the lesser plea and his trial continues.

Describing Jutting as highly intelligent, Latham said he began drinking heavily in 2010, often until he passed out. He began using cocaine intermittently in 2012 until his drug use escalated dramatically in the weeks prior to the killings.

Latham said Jutting became fixated on sexual violence and progressed from viewing extreme pornography to "acting out" fantasies with prostitutes. Then, the psychiatrist said, Jutting progressed to "non-consensual acts", culminating in what he described as "horrific acts".

The trial continues.