The Home Office is "refusing to protect" victims of modern slavery and human trafficking, a report has said.
A charity found some women were held at Yarl's Wood detention centre in Bedfordshire for months despite a policy to protect trafficking victims.
Women for Women Refugees said one woman was released to a place where she was previously "forced into prostitution".
A spokesperson for the Home Office said it had made "significant improvements" to detention in recent years.
For its report, Women for Women Refugees looked at the cases of 14 women from China who were all detained at Yarl's Wood Immigration Removal Centre.
The charity said in most of the cases the women had been brought to the UK and forced into sexual exploitation or forced labour to pay off a debt owed by themselves or family members.
Women for Women Refugees said the Home Office's "commitment to safeguard and protect vulnerable people is not being implemented".
In 2016 the Home Office introduced an Adults at Risk policy that aimed to reduce the number of vulnerable people in detention.
The policy stated vulnerable people, including survivors of trafficking, should not normally be detained and a "detention gatekeeper" should "challenge decisions" about who entered immigration detention.
'One hell to another'
Women for Women Refugees said this policy was not being adhered to as some detained women had been "encountered by policy during raids on brothels or massage parlours".
It said these were "clear, objective indicators" that they might have been sexually exploited or victims of trafficking.
"In spite of this, these women were not treated by the police as possible victims, and provided with help and support. Rather, they were referred to immigration enforcement and detained," the report said.
The charity said this demonstrated the Home Office was "deliberately refusing to protect women who have experienced serious human rights abuses".
One woman said being transported to Yarl's Wood was like being "taken from one hell to another".
After being contacted by women in detention at Yarl's Wood, the charity put them in touch with solicitors and all 14 women have since been released and are seeking asylum.
According to Home Office figures, women from China made up the largest group of female adults in immigration detention in the UK.
During 2018, 420 women from China were detained out of a total of 3,641.
A spokesperson for the Home Office 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.
"We continue to explore alternatives to detention, increase transparency around immigration detention, further improve the support available for vulnerable detainees and initiate a new drive on detainee dignity.
"Any person we encounter who claims they are a victim of trafficking will, with their consent, be referred to the National Referral Mechanism. Their claim will then be considered by a trained specialist."