A chilling image shows gaunt nanny Sophie Lionnet just days before she was murdered by a French couple at their London home, as the victim's mother branded her killers "monsters" who had "starved, tortured and broke" her daughter.

French au pair Sophie Lionnet was waterboarded and beaten to death at the Wimbledon home of Sabrina Kouider and her partner Ouissem Medouni after she was put through bizarre "interrogations" by the paranoid pair.

The couple were convicted of murder today following an often-disturbing trial at the Old Bailey, reports Daily Mail.

Speaking after the verdicts, Sophie's mother Catherine Devallonne said in a statement: "These monsters repeatedly beat Sophie. They starved her, tortured her and broke her until she could no longer fight.

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"We have now been tortured ourselves, forced to listen to Sophie's last moments, her sobs and her crying.

"Sabrina, no amount of shaking will ever cleanse your soul."

Ms Catherine Devallonne told the killers: "No one, no God will ever forgive you both for what you have done to our daughter.

"You will not be forgiven for the lies.

"My heart is broken, shattered into a million pieces. Sophie's loss will not be meaningless, lessons will be learned."

An often-disturbing trial has heard how Kouider was obsessed with her former lover, Boyzone founding member Mark Walton, and convinced herself the nanny had been hired by him to spy on her.

Miss Lionnet was completely innocent and had never met Mr Walton, but that did not stop Kouider making increasingly outlandish claims against the quiet 21-year-old, which turned into violent beatings.

The au pair did not have the money for the flight home to France and so became trapped in the couple's terrifying cycle of recriminations and violence before she died during a beating last September.

Kouider and Medouni blamed each other for the murder in a cut-throat defence but were convicted by an Old Bailey jury after a week of deliberations.

Kouider, who had ranted at her former lover throughout the trial, cried and shook her head in the dock before the verdicts were returned.

She was found guilty unanimously and burst into hysterics, saying 'no' and shaking her head.

Medouni was convicted on a majority of 10-2. He wept quietly and stared at the floor as the verdicts were read out.

Sophie's mother Catherine Devallonne had cried quietly to herself as she waited for the jury to come back.

Following the conclusion of the trial, Judge Nicholas Hilliard QC, told Sophie's parents: "I am absolutely sure that there was no truth whatsoever in the allegations that were made against your daughter."

Kouider carried on shrieking and shaking her head as the judge consoled the family.

In 2015 a friend of Kouider's brother Djamal suggested Miss Lionnet try out as Kouider's nanny, and in January 2016, a few days after her 20th birthday, she moved in permanently with the couple in London.

It was the first and only job she ever had and even though she was paid just £50 (NZD $96) a week she was constantly criticised by Kouider for being "lazy".

For over a month, Miss Lionnet was interrogated, beaten and threatened by the couple as she desperately tried to appease them with false confessions and apologies.

Sophie's mother Catherine Devallonne begged Kouider over the phone to send her daughter home, but Kouider said she would not do so until she learned "the truth" about Sophie and Mark Walton.

In the 48 hours before she died, Sophie suffered fractures to her breast bone and five of her ribs that were inflicted with severe force.

Her jaw was also smashed in one of the final attacks, although the precise cause of her death may never be known. Pathologists believe she may have been drowned or suffered a blow to the head.

For two days after she died, the couple kept the body in a suitcase in their shed, uncertain what to do.

But the couple's attempts to cremate her body on the garden bonfire using nail polish remover as an accelerant meant that her corpse was too severely burnt for a precise cause to be established.

They were also about to set fire to a carrier bag full of Sophie's written apologies, confessions and pleas for help when the fire brigade arrived.