People smugglers are charging Vietnamese nationals more than £30,000 to reach the UK - where they are told they will live like royalty, a new report reveals.
It describes a two-tier pricing system, including a "premium" service costing up to £33,000 which promises as direct a route as possible to Britain with minimal risk.
Research suggests smugglers also offer "economy" options which range in price from £10,000 to £20,000.
Migrants choosing the cheaper service can face journeys of several months as they make their way across Europe.
The findings are detailed in a report on the exploitation of Vietnamese nationals en route to and within the UK commissioned by the Independent Anti-Slavery Commissioner Kevin Hyland.
The report found:
Between 2009 and last year, 1,747 Vietnamese nationals were identified as possible victims
31 of 70 victims had experienced modern slavery en route to the UK
China, Russia, France and the Czech Republic were the most common countries where victims were exploited
The report describes irregular migration as "big business" and points to evidence that smuggling agents in Vietnam exaggerate the reliability of their services and the financial rewards attainable in Britain.
The paper cites an account given by one interviewee who paid US$25,000 (just under £19,000) to people smugglers.
The unnamed interviewee said she was promised a life of luxury in the UK, but the reality was far different.
I was told that I would live like a queen in the UK, good food, nice clothes, easy job with a high salary.
The report also suggests that smugglers are attempting to exploit arrangements for dealing with visa applications.
It notes that applications from Vietnamese nationals are processed at the British Embassy in Bangkok.
"These decisions are taken under time pressure and it is thought that smuggling groups are aware of this and are using the premium visa services as they think that these applications will not be fully scrutinised," the assessment says.
"Legitimate visas are also targets for agencies who pursue Tier Four visas for study groups to the UK and then arrange for the students to disappear and for smuggling groups who use legitimate passports to claim visas for similar-looking Vietnamese nationals."
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Figures show that between 2009 and last year, 1,747 Vietnamese nationals - including hundreds potentially exploited as minors - were referred to a service set up to identify possible victims.
The new analysis found that 31 of 70 victims for whom it was possible to identify a country or point in their journey when they had been exploited had experienced modern slavery en route to the UK.
China, Russia, France and the Czech Republic were the most commonly identified countries in which victims were exploited.
The report emphasised that Vietnamese modern slavery is "not only an international issue but a domestic one too", with UK-based exploitation evident in 55 cases.
Researchers found that the most common sector within which Vietnamese nationals experience modern slavery in the UK is labour exploitation - including work in cannabis cultivation and nail bars, followed by sexual exploitation.
The study makes a number of recommendations including measures to regulate the nail bar industry, saying: "This kind of business and others considered high risk could be required to prove compliance with current regulations as part of a licensing scheme."
It also raises concerns that some Vietnamese individuals who go on to be recognised as victims of forced labour are criminalised first.
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