A French couple was on Thursday found guilty of murdering their 21-year-old au-pair, whose burnt body was found in their London garden in September last year.

Sabrina Kouider, 35, broke into tears after she and partner Ouissem Medouni, 40, were found guilty by a jury at London's Old Bailey court of murdering French live-in nanny Sophie Lionnet and burning her body on a garden bonfire.

"Only Kouider and Medouni know exactly how they killed Sophie but the prosecution was able to prove that she died as a result of purposeful and sustained violence, and not by accident," said state prosecutor Aisling Hosein.

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"They were both jointly involved and came up with a plan to try and destroy her body and escape responsibility for this horrendous crime for which they have been found guilty," she added.

The jurors deliberated for a week before unanimously convicting Kouider and ruling by a majority decision of 10 to 2 that her partner Medouni was guilty.

Kouider is to undergo psychiatric assessment before sentencing, which is due to take place on June 26.

Both denied murdering Lionnet, although they had admitted burning her body in their garden.

The court heard how the couple had interrogated and tortured Lionnet over their belief she was conspiring with one of Kouider's former boyfriends -- Mark Walton, a former member of Irish band Boyzone -- who they claimed sexually abused members of their family.

Kouider told London's Old Bailey court that she hit the au pair "really bad" with an electrical cable.

They both accused each other of killing her.

In her testimony, Kouider said her partner -- who she called "Sam" -- was violent to her and "was with me for sex, nothing else, and money".

Their lawyers have 30 days to appeal the decision.