Rita lives in London and works seven days per week. She rises at dawn and often doesn't get to bed until the small hours. Each day, she washes and cooks for three children, then cleans the house of her employer from top to bottom. Rita is fed on scraps of food, and sleeps in what is best described as a glorified cupboard, with a thin mattress on the floor. The mother of the household frequently verbally and even physically abuses her. The father, a wealthy businessman, has made occasional sexual advances. When Rita arrived in the United Kingdom from Indonesia, her employer also confiscated her passport. And she is still waiting for her first paycheck.

By most definitions, Rita's experience should count as modern day slavery. But thousands of foreign domestic workers are in the United Kingdom right now, open to abuse because the present government has introduced a tied visa system, in which foreign domestic workers' immigration status is entirely dependent on their employer. If their employer happens to be abusive – economically, physically, sexually or verbally – the domestic worker cannot move to a new household. She (foreign domestic workers are mainly women) is trapped. This is the much disdained kafala system of the Gulf, alive and well here in the UK. This is modern day slavery.

Yet Theresa May and the Home Office think otherwise. Her government has continually blocked moves to repeal these immigration laws, introduced from April 2012 – even as human rights groups, domestic worker support groups and the domestic workers themselves warn they are having devastating effects. An amendment to May's own Modern Slavery Bill, asking for the tied visa system to be repealed, was recently rejected after the Tories rallied to vote against it.

According to Kalayaan, a charity that works to protect abused domestic workers in the UK, 16% of those who arrived after the tied visa system was implemented report physical abuse, compared to 8% prior to its introduction. 65% on the tied visa don't have their own bedrooms, often sleeping in the kitchen or lounge – compared to 34% previously. Four fifths of tied workers have had their passports confiscated. There are reports of sexual abuse within foreign embassies, of billionaires from the Gulf or south Asia, living in west London mansions, who do not pay their domestic workers and beat them if they “misbehave.” One employer threatened to throw a domestic worker of the top of a sixth storey building.

Leaving these abusive employers puts the domestic workers in an impossible situation. They are then unable to work legally – putting them at even more risk. One woman, unable to find work, was forced to live in a central London park, where she was raped. Although the government claimed, in rejecting the amendment to the Modern Slavery Bill, that laws already exist to protect these abused workers - in reality, when workers have reported themselves to police stations, they are quickly deported. Back home, they often have large families to support, or ill family members – and their salary in the UK is their only chance of feeding them.

Foreign domestic workers, on the whole, are well treated. But there are too many cases of terrible abuse. The Modern Slavery Bill is a great opportunity to repeal the barbarism of the tied visa system. Theresa May and her Tory government should recognise that. She wants to look tough on immigration but the suffering of thousands of women is too terrible a price to pay. As the Bill moves forward into the Lords, it is imperative these laws are repealed.

Alastair Sloan, unequalmeasures.com