Rurik Jutting, a Cambridge graduate who went to one of Britain's top public schools, is accused of torturing and murdering two Indonesian women in his Hong Kong flat.
He's denied murdering Sumarti Ningsih, 23 and Seneng Mujiasih, 26, despite their bodies being found in his apartment.
The judge has warned jurors the case would be dealing with "extreme cruelty and violence", and that photographs of what he allegedly did to the victims may be shown in court.
The prosecution and defence are believed to agree what happened, but disagree over Jutting's mental state.
Jutting, who wore a dark blue shirt, glasses and looked a lot slimmer than in his court appearances last year, was put into a glass-screened dock when he arrived in the court.
When the clerk asked what his plea was to the two murder charge, he replied "not guilty to murder by reason of diminished responsibility but guilty of manslaughter", which the prosecutors refused to accept, meaning the trial on the murder charges will proceed.
A third charge was also read out, unlawful burial of Ms Ningsih's body, to which he pleaded guilty. Her body was found stuffed in a suitcase left in a balcony while Seneng Mujiasih's body had knife wounds on the neck and buttock, according to initial police reports.
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