Vietnamese children rescued from traffickers in Britain are vanishing from local authority care, with many feared at risk of re-enslavement by criminals, charities said on Friday.

More than 150 Vietnamese minors have disappeared from care and foster homes since 2015, with almost 90 others going missing temporarily, according to the Times newspaper.

Anti child trafficking organization ECPAT U.K. said its own research showed 28 percent of all trafficked children in care went missing at least once, with Vietnamese children the most likely to abscond.

“It’s a scandal that there are (so many) missing children,” said Chloe Setter, ECPAT U.K. head of advocacy.

“It’s going on in every part of the country, but it’s a hidden issue,” she told the Thomson Reuters Foundation.

Vietnam consistently ranks as one of the top three source countries for potential victims of modern slavery in Britain. Data suggests just over half are minors.

Victims commonly end up in labor exploitation, often in cannabis cultivation and nail bars. Some are also sexually exploited.

Experts say rescued Vietnamese children may abscond from care because they do not feel safe or feel isolated, particularly if they do not speak English.

Some may also contact their trafficker after being rescued because they fear reprisals against themselves or their families back home, because they believe they are in debt-bondage or because their trafficker has made false promises.

Many vanish within days of being placed in care, experts said.

Britain’s Independent Anti-Slavery Commissioner Kevin Hyland expressed alarm at the number of disappearances in a recent report on trafficking from Vietnam.

He highlighted the case of a Vietnamese boy who was re-trafficked while in foster care after being found at a cannabis farm.

After his arrest at a second cannabis farm, the boy was placed back in care, but was trafficked again into domestic servitude, which included sexual abuse.

The government said it was introducing a scheme to give trafficked children specialist advocates or guardians who could provide support and reduce re-trafficking risks.

“We have strengthened regulations on children’s homes and placed a duty on local authorities to tell us about all incidences of children going missing from care, even those lasting less than 24 hours,” a government spokesman added.

But Setter said there was a need for specialist foster carers to be trained in looking after trafficked children. Another expert called for the establishment of safe houses with high levels of support and supervision.