Independent Anti-Slavery Commissioner Kevin Hyland warned yesterday that “irregular migration” is big business, with people-smugglers charging between £10,000 and £33,000 per person.

But they exaggerate the reliability of their services and the financial rewards attainable, he added.

Agencies were also found targeting legitimate visas for study groups to Britain and “arranging for students to disappear.” Smuggling groups then used legitimate passports to “claim visas for similar-looking Vietnamese nationals.’

Between 2009 and last year, 1,747 Vietnamese, including hundreds potentially exploited as minors, were referred to a service set up to identify possible victims. Half were found in Britain, while the rest were in China, Russia, France and the Czech Republic.

Vietnamese nationals have experienced modern slavery here cultivating cannabis, working in nail bars or as prostitutes.

The report also raised concerns that while some people who have been smuggled go on to be recognised as victims of forced labour, they are often criminalised first.