A woman who claimed she was beaten and sexually assaulted by a diplomat and his royal wife, who brought her to London, has spoken of her fear of reprisals.

The 23-year-old, who has been granted refugee status, is owed more than £20,000 following an employment tribunal order, but has been unable to get justice as the couple have returned home.

The woman said she had been treated as little more than a slave after having been promised paid work as a nanny, a room in the couple's magnificent house and days off to enjoy London. Instead, she alleged, they violently beat her, sexually assaulted her and locked her away to work 19-hour days for no pay.

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The couple, whose identity is known to the IoS, cannot be named as the victim fears reprisals. She said that they had had her followed after she escaped from their home last year, and have subjected her and her family to death threats.

Her tale of modern slavery is one that is all too familiar in Britain. There are more than 20,000 migrant domestic workers estimated to be in the UK. Those working for diplomats are particularly vulnerable because visa restrictions prevent them changing employer. Last year 189 visas were issued to domestic workers accompanying diplomats, and every year some of these will be subjected to abuse. Now campaigners are demanding a change in the law.

Although the woman reported her allegations to police, they advised her that the couple could not be prosecuted because of their diplomatic status. The Foreign and Commonwealth Office (FCO) contacted their embassy but were told that since they have returned to their home country they cannot be chased for the compensation payments ordered in January.

The domestic worker was 21 when she came to Britain. "From the first night I knew something was wrong," she said. "I was made to share a room with the diplomat and he came into my bed and touched me all over.

"I was so scared but I spoke no English and had no money and no phone. I was trapped. I was paid nothing, never allowed to leave the house, and only given scraps to eat.

"They made me get up at six to cook, clean and care for them and their children; I didn't get to bed until one in the morning. They treated me like dirt, throwing things at me, shouting at me and hitting me ... I hand-washed all their clothes until my hands were inflamed. If I didn't do what they asked they would beat me and smash my head against the wall. Every time I asked to go home they threatened me. They said they would destroy my passport and harm my family. I was terrified because I knew they could; they have power in my country."

Kalayaan, a charity dedicated to helping migrant domestic workers, says her case illustrates the terrible abuses that diplomats can get away with in this country. Jenny Moss, a community advocate for the charity, said: "To protect diplomats' domestic workers it is imperative that the UK Government give them the basic right to change employer, so they can flee abuse and access protection and justice without fear of deportation. The power that employers can wield over vulnerable individuals is akin to bonded labour and leads to human trafficking."

An FCO spokesman said: "We expect foreign missions and diplomats to meet their obligations under the Vienna Convention on Diplomatic Relations 1961, to respect the laws and regulations of the UK, including employment rules."