Hundreds of victims of trafficking have been locked up in detention centres by the Home Office instead of being looked after in safe houses and provided with a package of support, the first research into the extent of the problem has revealed.

The report, carried out by the data mapping project After Exploitation using freedom of information responses, reveals that 507 victims of trafficking were detained in 2018 despite Home Office guidance that this group should not normally be locked up.

The 507 victims were detained last year, even though the Home Office had made a “reasonable grounds” decision about their cases confirming they were suspected victims of trafficking, which should normally entitle them to a package of support including accommodation in a safe house, money and a support worker.

Last year, 2,726 suspected victims of trafficking were identified, meaning almost one-fifth were put in detention. It is understood that of the 507 who were detained, 422 were released within a week of a positive “reasonable grounds” decision being made.

A separate piece of research by the charity Women for Refugee Women examined the cases of 14 Chinese victims of trafficking who were detained and found that some were taken straight from immigration raids on brothels and massage parlours to detention, despite clear indications they had been sexually exploited.

Theresa May has described modern slavery as the greatest human rights issue of our time, but the After Exploitation study, titled Supported or Deported, accuses the government of a “disturbing” failure to protect victims of trafficking.

The Women for Refugee Women research accuses the Home Office of breaching its own policies not to detain vulnerable victims of trafficking. According to Home Office data Chinese women made up the largest group of those detained in Yarl’s Wood detention centre in Bedfordshire last year: 420 women.

The report – From One Hell to Another: the Detention of Chinese Women Trafficked to the UK – examines 14 case files of Chinese trafficked women held in Yarl’s Wood represented by Duncan Lewis solicitors and finds that most were brought to the UK to pay off a debt and then forced into domestic or sexual slavery.

Nine of the women were forced to work in brothels or massage parlours and the other five were made to work in restaurants or other forms of forced labour. The report calls for an end to detention in general and for cases of trafficking victims to be decided while they live in the community.

A trafficked Chinese woman held in Yarl’s Wood detention centre, speaking anonymously, said: “The gang leaders forced me to do things that I didn’t want to do, Then one day men in uniforms came to the house. I was terrified and tried to hide but they found me. They dragged me out and took me to the police station … and I ended up at Yarl’s Wood. I was taken from one hell to another.”

The After Exploitation report calls for automatic release from detention when people are suspected of being a victim of trafficking and urges the Home Office to provide transparent reporting of outcomes after contact with the national referral mechanism, the framework to which suspected and confirmed victims of trafficking are referred.

Maya Esslemont, the director of After Exploitation, said: “The unjustifiable detention of potential trafficking victims shows disturbing failings by the government to protect vulnerable people from prison or prison-like settings under immigration powers.”

Pierre Makhlouf, an assistant director at Bail for Immigration Detainees, said: “These figures confirm what we have long suspected: that the Home Office knowingly detains victims of trafficking on a large scale.”

The Labour MP Jess Phillips is due to lead a Westminster Hall debate on the issue at 2.30pm on Tuesday.

A Home Office spokesperson said: “Detention is an important part of the immigration system – but it must be fair, dignified and protect the most vulnerable. We have made significant improvements to our approach in recent years, but remain committed to going further.

“Any person who claims they are a victim of trafficking will, with their consent, have their claim considered by a trained specialist and will not be required to leave the country while this decision is pending. A positive decision entitles that person to support and guidance and is taken into consideration when deciding their immigration case.”