Some diplomats to the UK are alleged to be abusing their immunity from the law and employing victims of human trafficking as domestic workers in inhumane conditions without pay, a Sky News report has revealed.

Migrant workers, doing domestic work in embassies in London, or working for the families of diplomats, said they are being kept in modern day slavery and obliged to work for free for long shifts in appalling conditions.

Speaking to Sky News, one domestic worker spoke of her regime, being virtually imprisoned in the home of her employer's.

"Sometimes I would have to wake up and start working at five or six in the morning and I would finish around 8pm," said Dian, whose name has been changed to conceal her identity.

"But sometimes I had to look after the children until midnight. I did ask for my salary but they kept saying 'later, later'. They never paid."

Dian, who was working to repay the debt she owed traffickers for bringing her to the country, eventually fled.

"I left while they slept. It was getting colder and colder. It was winter and I only had my flip-flops. Now I am happy although still I often have nightmares about what happened to me," she said.

The embassy where the diplomats work was contacted by Sky News but refused to comment.

One charity which works with victims of trafficking found that of the 120 domestic workers it surveyed who worked for diplomats or were otherwise tied to their employers, 69% had potentially been trafficked, Sky reported.

Out of those questioned, 71% were not allowed outside, 79% had no time off and 29% were not paid at all.

"Diplomatic domestic workers are a very vulnerable group," said Emily-Anna Gibbs from the Anti Trafficking and Labour Exploitation Unit, who is currently representing two former domestic workers against diplomats from Saudi Arabia.

"Their employers are immune. We know that many employers do use that immunity as a shield for very serious abuses that happen behind the doors of their diplomatic homes."

According to official global estimates, there are between 20 and 30 million men, women and children trapped in slavery all over the world.

The number includes victims of human trafficking, sexual exploitation, child labour, forced marriage, and forced recruitment for use in armed conflict.