A British banker was using cocaine when he killed two Indonesian women and then used his phone to film himself talking about it, a Hong Kong prosecutor said as a trial for the 2014 killings opened on Monday.

The banker, Rurik Jutting, entered a plea of not guilty to two murder charges that were read out at the High Court, with prosecutors rejecting his attempt to plead guilty to the lesser charge of manslaughter.

Jutting is charged with the murders of Sumarti Ningsih, 23, and Seneng Mujiasih, 26, whose bodies were found in his apartment near the Asian financial center's Wan Chai red-light district.

In opening remarks, Prosecutor John Reading told jurors that Sumarti went home with Jutting after he offered her "a large sum of money" on Oct 25, 2014.

Jutting subjected her to "increasingly cruel acts of violence using his belt, sex toys, a pair of pliers and his fists", Reading said.

After torturing her for three days, he took her into the bathroom and cut her throat with a serrated-edged knife, Reading said.

Jutting used his phone to film himself talking about the killing, how he enjoyed dominating Sumarti and how he watched pornographic videos involving extreme violence. He also said he "definitely could not have done that without cocaine."

On Oct 31, 2014, Jutting took Seneng back to his apartment. She was officially in Hong Kong as a foreign maid, but was working at a bar when Jutting offered her money for sex, Reading said.

After they undressed, Seneng spotted a gag made of some rope that Jutting had left next to the sofa and started to shout. Jutting grabbed a knife and held it to her throat, telling Seneng he would cut her throat if she didn't stop, Reading said.

"She continued to struggle and shout, and he cut her throat," he told the jurors.

Reading said traces of cocaine were detected in more than two dozen small plastic bags found in Jutting's apartment.

When the clerk asked what his plea was to the two murder charges, Jutting 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.

The judge told jurors that they could still decide between finding him guilty of murder or manslaughter.

A third charge was also read out, unlawful burial of Sumarti's body, to which Jutting pleaded guilty.