Exposure: The truth about Britain's secret slaves

Watch Exposure: Britain's Secret Slaves tonight on ITV at 10.40pm.

The young woman sitting in front of us speaks softly. In English, heavy with a Filipino accent, she tells us her story.

And then her grim assessment of her life in Britain as a migrant domestic worker, employed by a wealthy foreigner who brought her with his family to London.

That any worker should suffer such treatment here in 21st century Britain is scandal enough.

But it's happening at a time when the Government is staking claim to be a world leader in dealing with trafficking and slavery with its Modern Slavery Bill, currently going through Parliament. Home Secretary Theresa May has said tackling such evil is her personal priority.

Many aspects of the Modern Slavery Bill have been warmly welcomed, and indeed garnered praise around the world.

But when it comes to the treatment of domestic workers, legitimately brought here by rich foreigners and diplomats, the Government is being accused of having a wilful blind spot - because of its attitude to immigration.

On tonight's Exposure programme, one specialist lawyer says this has made the Government effectively complicit in modern slavery.

In 2012, as part of the clampdown on immigration, the visa rules for foreign domestic workers were changed.

The old system allowed them to change employers once they were brought here, giving them a straightforward escape route if they experienced bad treatment. They could find a new job and apply to get their visa renewed. This system was praised as a model of fair play. The UN described it as "instrumental in facilitating the escape of migrant domestic workers from exploitative and abusive situations".

But after 2012 that all changed. The workers were to be effectively tied to the employers who brought them here. It meant they had no legal right to leave their job. Now, if they do escape they are in immediate breach of immigration law. Many are too scared to go to the police and disappear onto the black market where they're vulnerable to yet more exploitation. Those who are employed by diplomats know their employers are practically untouchable in law.

"It's basically creating a pool of workers here who are ripe for exploitation, at a time when we're saying we want to combat modern slavery in the UK," says Kate Roberts from Kalayaan, which helps migrant domestic workers and campaigns for their rights. She tells me:

Be in no doubt what these women are suffering is slavery. Some of the stories I have heard in the last few months take your breath away. Long hours on pitiful or non-existent pay is only the start of it.

These women are supposed to be protected by British law while working here. Yet they are being starved, physically and verbally threatened, spat at and humiliated. Some are locked inside some of the most beautiful mansions in London. Those who are allowed out are often held with invisible chains, their lives threatened, their passports confiscated by their employers.

The Home Office says such workers are briefed on their rights in the UK before they come here, by British officials here and abroad.

But not one of the many women we spoke to for Exposure had been given any information.

Our programme tonight lifts the lid on the suffering of migrant domestic workers, here on our own doorsteps. We hear stories of the brave women who've managed to escape - but also the secret diary of one who is too scared yet to run away, employed in a mansion in London. We expose a sham agency which offers to arrange fake passports and low paid work for those who do get out and run to the black market in desperation.

And as pressure mounts on the Government to act and change the visa system, we hear the reasons why they're refusing to do so.

Watch Exposure: Britain's Secret Slaves tonight on ITV at 10.40pm.