The alleged torture and murder of a French nanny over a bizarre fantasy involving a former Boyzone pop star is 'stranger than fiction', a prosecutor has said.

Sabrina Kouider and Ouissem Medouni treated 21-year-old Sophie Lionnet as 'less than human' when they allegedly imprisoned, beat and killed her at their flat, the Old Bailey heard.

The couple, who threw the body on a bonfire, were driven by a 'preposterous' obsession with Kouider's ex-boyfriend Mark Walton, a wealthy singer-turned-music mogul, the court was told.

In his closing speech, prosecutor Richard Horwell QC said: "Of all the cases this historic building has heard, this must without hesitation enter the category of the more bizarre.

"Expressions such as 'you really could not make it up' and 'truth is stranger than fiction' come readily to mind.

"The defendants made a truly odd couple. There is a unique bond between them that has kept them together on and off for many years, a bond based partly in love and something close to it.

"But as far as this trial is concerned the point that really matters is that together they were a truly toxic combination."

Miss Lionnet was an 'ingenue if ever there was one' when she stepped into their "turbulent and unconventional" lives, Mr Horwell said.

Unbeknown to her, the defendants made for an 'unhealthy and dangerous mix', he said.

In contrast, kind-hearted Ms Lionnet was 'malleable, compliant and submissive' and in the wrong hands 'ripe for exploitation', he said.

She was caught up in the defendants' "extraordinary fantasies" about Mr Walton, which were described by the lawyer as "truly preposterous".

The defendants' 'unhealthy, myopic, all-consuming and groundless' obsession 'deprived them of reason' and cost Ms Lionnet her life, the court heard.

Mr Horwell said the couple tortured Ms Lionnet in the bath and killed her out of 'revenge and punishment'.

The defendants saw her as 'expendable' as she had already confessed on video to her association with Mr Walton and could not take it back if she was dead, the court heard.

The lawyer hailed LA-based Mr Walton's 'integrity and honesty' in voluntarily coming forward to give evidence.

He told jurors: "Walton is a wealthy man - and good luck to him for that - but it is of course a truth universally acknowledged that a single man in possession of a good fortune will often be parted from it."

Mr Horwell suggested the allegations against the Pop Idol Vietnam judge began after his generosity towards Kouider and her family dried up.

Kouider, 35, and Medouni, 40, have denied murdering Ms Lionnet at their home in Southfields, south west London, in September last year, and have blamed each other for her death.

The pair have admitted perverting the course of justice by disposing of the body in their back garden.