A designer murdered a 21-year-old French nanny and burned her body after accusing her of "outlandish" crimes she had not committed, the Old Bailey heard today.

Sabrina Kouider, 35, the ex-girlfriend of Boyzone founder Mark Walton, allegedly killed "naive and vulnerable" Sophie Lionnet after a "brutal and oppressive inquisition".

Kouider and partner Ouissem Medouni, 40, are said to have held Lionnet prisoner at their £900,000 ($1.7 million) home in Wimbledon, Southwest London, in the days before her death, the Daily Mail reports.

The court heard Kouider accused her nanny of being "in league" with Walton and in rambling complaints said he controlled her through "black magic".

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Richard Horwell QC, prosecuting, said: "The last days and hours of Sophie's life must have been truly wretched. She was subjected, at times, to a brutal and oppressive inquisition and to significant violence."

Lionnet died after suffering fractures to her sternum, four ribs and her jawbone. Kouider and Medouni then allegedly tried to cover up the murder by burning her body in their garden.

Horwell said: "The plan was to dispose of Sophie's body and to explain her disappearance by inventing a story that she had left under something of a cloud and had returned to France.

"Another missing person, no longer their responsibility, her disappearance nothing to do with them."

Their plan was foiled when a neighbour became concerned about the fire and dialled 999, jurors were told.

Lionnet's charred body was discovered at the house on September 20 last year.

The young victim, from Troyes, northeast France, had moved to the UK in 2016 to improve her English.

"Sophie was not only young, but also naive and particularly vulnerable and this made her an easy target for abuse and exploitation," said Horwell.

"Sophie had a big heart but was not worldly wise and it was easy to take advantage of her."

Lionnet's health began to deteriorate before her death because of her "bizarre and oppressive" life, the court heard.

"At times she appeared scared and hungry," said Horwell. "She complained that she was being beaten and that she was not allowed to return to her home in France."

Kouider was allegedly seen shouting and screaming at Lionnet. She accused the young Frenchwoman of stealing a diamond pendant and plotting against her, the court heard.

Horwell said: "Precisely what was in Sabrina Kouider's mind may be difficult to determine but it seems that the more outlandish the allegations the more she pursued them, despite the fact they were denied and despite the fact there was no evidence to support them.

"Her allegations appear to have been contagious because Medouni was beguiled by Kouider and her obsessions and delusions and he began to adopt them himself.

"Eventually they confronted Sophie and wanted her to confess to conduct and crimes she had not committed. Sophie became a prisoner in the Wimbledon home and she must have been terrified.

"The defendants mistreated and intimidated Sophie in a manner that is way beyond anything that could be considered normal or rational.

"Sophie must have found this unnatural and increasingly toxic situation wholly outside her experience and ability to manage. At times she made confessions to please her oppressors and then would withdraw them.

"And so the pressure increased and so did the violence against her."

Jurors were told Kouider had been in a relationship with Walton.

Horwell described Walton as "an original member of the successful Irish boy band Boyzone" who now lives in Los Angeles.

Kouider started to rent the ground floor flat in August 2013.

Walton paid for the first month's rent and deposit of more than £4000 ($7700) and another five or six months' rent, jurors were told.

She later "got into significant rent arrears" after the payments stopped.

Kouider worked from home as a fashion designer but was "not particularly successful" the court heard. She got to know Lionnet from January 2016.

The victim had studied a vocational training course in child care and wanted to work with children. She last saw her mother at around the time of her 20th birthday.

The court heard firefighters discovered Lionnet's remains under the ash of the fire in the garden.

"Without such vigilance the two defendants might well have got away with murder, which was, of course, their aim," said Horwell.

The prosecutor told jurors that it was understandable if the case appeared "difficult to believe at first blush".

"How can someone be treated in such an appalling manner and not leave?" he asked. "How can someone confess to fantastical allegations which are not true?

"It is likely that never before will you have encountered a combination of individuals such as these two, Sabrina Kouider and Sophie Lionnet."

Kouider was "vindictive, overbearing and controlling" while Lionnet was "timid, uncomplaining and especially vulnerable to manipulation and threats".

Horwell said it was "a unique and highly dangerous combination that led to murder and the attempted disposal of a body".

Kouider was "obsessed with Mark Walton and complained that he was stalking her", according to one witness.

The prosecutor said the "love between them ended some five or so years ago".

"On Kouider's part, that love soon turned to anger and bitterness," added Horwell. "You may conclude that it turned into a form of obsession."

Lionnet allegedly confessed she was being "beaten" by Kouider to a local fish and chip shop owner.

She regularly visited the takeaway, often dressed in the same clothes, to eat chips at speed as if she was hungry, the court was told.

Lionnet said she wanted to go home to France because her mother was unwell but was not allowed.

In August 2017 Kouider yelled at Lionnet in a high-pitched voice and called her a "bitch", it is claimed. Kouider was also seen by a friend accusing Lionnet of stealing a diamond ring, the court heard.

The same witness was present when Kouider knocked Lionnet to the floor during a row in the summer of 2017, it is claimed.

"Sophie was on the floor, holding her head and crying," said Horwell. "She had obviously been assaulted."

Kouider was in a "total rage", the court heard. The prosecutor said Lionnet was "trapped in a nightmare and tragically there was much worse to come".

Medouni and Kouider, of Wimbledon, Southwest London, deny murder. The case is expected to last for up to five weeks. The trial continues.