Sabrina Kouider and Ouissem Medouni had kept 21-year-old Sophie Lionnet as a "prisoner" and violently abused her after she moved into their southwest London home, according to an indictment read in court.
Designer Kouider, 35, dressed in black, and her partner Medouni, 40, wearing a dark suit, pleaded not guilty to killing Lionnet, who was originally from Troyes in northeastern France.
Medouni has pleaded guilty to perverting the course of justice.
The couple were arrested after Lionnet's charred remains were found in their garden on 20 September, shocking the community of young French au pairs in the British capital.
Police had been alerted by neighbours who spotted thick smoke coming from a back garden in the well-to-do neighbourhood of Wimbledon.
"Without this vigilance, the two accused could have escaped with impunity despite their murder," said prosecutor Richard Horwell.
"The defendants burnt her body in the garden of their home in the hope that no one would ever discover her remains."
Lionnet had suffered fractures to her sternum, ribs and jawbone, but the exact cause of her death was unknown due to the attempt to dispose of her body.
- with AFP