More than 500 suspected slavery victims were held in immigration detention centers by British authorities last year, according to official data released yesterday that experts said showed survivors were being treated as criminals rather than victims. Immigration officials were failing to adequately support those escaping slavery and risked adding to their trauma by locking them up, found a report from After Exploitation, which used Freedom of Information requests to obtain the data.

"All too often victims of trafficking are perceived as immigration offenders rather than victims," said Pierre Makhlouf, assistant director at the charity Bail for Immigration Detainees, in response to the report.

"Detention is an important part of the immigration system – but it must be fair, dignified and protect the most vulnerable," a spokeswoman told the Thomson Reuters Foundation. "We have made significant improvements to our approach in recent years but remain committed to going further."

Britain was home to at least 136,000 modern-day slaves in 2018, according to the Australian human rights group Walk Free Foundation, a figure about 10 times higher than a 2013 government estimate.

People who are recognized as likely victims of modern slavery are entitled to a package of support including housing, counselling and some living costs while their case is assessed under Britain's National Referral Mechanism (NRM) scheme. However, victims of trafficking do not have any automatic entitlement to remain in Britain, and they can be detained in some circumstances in order to carry out their removal.

Charities providing support in detention centers report that vulnerable people often lack adequate access to health care, mental health support and legal services, the report said, putting slavery victims at risk of further harm. The Home Office said 479 people were already in detention when they were officially recognized as probable slavery victims, and nearly 90 percent were then released within a week. The data showed officials had either allowed known potential victims to be detained or had failed to recognize the signs of trafficking, the report said.

The number of human trafficking victims is on the rise globally and children account for 30 percent of all detected victims, with far more girls detected than boys, according to a U.N. report released in January. The U.N. Office on Drugs and Crime (UNODC) said the overwhelming number of detected victims of trafficking globally is women, with a further 23 percent being girls, and the report warns that their share of the total is increasing.