A modern-day slave passed between three middle-class families during years of horrific abuse is suing a police force that returned her to one of her tormentors.

The terrified and illiterate Indian victim, who was beaten, raped and given out of date food that made her ill, made desperate pleas for help to Hertfordshire police as well as charities and other state agencies. But when officers from the force spoke to her, one of her powerful and well-connected abusers was used as an interpreter.

After she was handed back to her slave master, the victim was attacked, threatened with a knife and told she would be buried in the back garden of their multi-million pound luxury home for spoiling the family name.

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The treatment of the 40-year-old woman can be revealed today as three people – a wealthy optician, a butcher and a secretary – were convicted of her abuse that spanned more than three years. During that time that she was passed like a chattel between the families. She was kept a prisoner, given virtually no money and had her passport confiscated. However, when she fled, her pleas went ignored by police and other organisations on at least 12 occasions, according to court documents.

The woman's ordeal ended only after she was taken in by a migrant workers' charity and human rights' group Liberty took up her case.

"Various state agencies failed her… ignoring her repeated pleas for help, not adhering to their own investigative practice and it could be said ignoring the obvious," Caroline Haughey, counsel for the prosecution, told Croydon Crown Court.

The woman, described as one of society's most vulnerable people, came to Britain in 2005 to try to make a better life and to send money to her family in Hyderabad. But when she sought help she was threatened by her keepers, rebuffed or failed to make herself understood by authorities. In one case, a professional interpreter told a police officer investigating the case: "She's telling a lot of lies – it's common in her country," the court heard.

Police investigated while she was with three different families but critics say that the incidents did not appear to have been connected.

She was first taken to hospital in 2006 with a gashed foot after her 'employer' Shamina Yousuf, 33, hurled a cup at her. However, no action was taken after she was bullied her into not pursuing matters. She fled after more than two years but returned to work for other relatives of the family to try to secure the return of her passport.

She stayed in a one-room flat in St John's Wood making sandwiches in the shop below for £2-an-hour. She was raped by the husband, butcher Enkarta Balapovi, on several occasions.The woman finally moved to the home of an acquaintance, Shashi Obhrai and her IT consultant husband Balram, who lived on a private estate in Middlesex. She was forced to work seven days a week, 17-hours-a-day, cooking and cleaning for eight family members. .She escaped and her case was passed to Scotland Yard's trafficking unit which brought her abusers to justice.

Obhrai, 54, of Moor Park, Middlesex and Yousuf, 33, of Edgware, north London, were both convicted yesterday of assault. Obhrai, an optician, was additionally convicted of threats to kill. Balapovi, 54, of St John's Wood, northwest London, was convicted of rape by a jury at Croydon Crown Court. They will be sentenced next month. Two other defendants were acquitted. The victim, who cannot be named for legal reasons, has been left in a wheelchair in part because of the injuries sustained at the hands of her abusers. She has lodged a claim against the Hertfordshire force which initially investigated the case.

Corinna Ferguson, legal Officer for Liberty, said: "This trial may not have taken place at all had Liberty not reminded the police of their obligations towards victims under the Human Rights Act." Hertfordshire police said it would be "inappropriate to comment" while elements related to the case were still subject to criminal and civil process.