The number of suspected human trafficking victims in the UK has increased by almost 40 per cent in the last year as crime gangs exploit EU free movement rules.

In new official figures, 3,266 immigrants from 102 countries - a third of them children and more than half women – were deemed as victims of human trafficking.

Hundreds and possibly thousands more are believed to remain undetected by authorities and are surviving within the black economy living in substandard accommodation.

The National Crime Agency is investigating reports that organised crime gangs from Romania are involved running begging and petty crime gangs around London and other major British cities

Criminal gangs have been smuggling in people from Vietnam to operate cannabis farms, file photograph

The figures represent a 40 per cent rise on the 2,340 cases recorded in 2014. Many of those smuggled into the country and being sold into prostitution, slave labour and exploiting the benefit system.

The National Crime Agency is investigating five cases of organ harvesting, with three involving children.

Also, 105 children are believed to have been smuggled into the country to be sexually abused, up by some 70 per cent on the previous years.

Investigators believe that Britain is being targeted by international criminal gangs who are exploiting the free movement of goods and people within much of the EU to target Britain.

The Albanian mafia is suspected of smuggling 394 adults and 206 youngsters for sex, slave labour and domestic servitude.

A further 478 victims are believed to be from Vietnam, an increase of 121 per cent on 2014. More than half of the men and boys are understood to have been trafficked to work on illegal cannabis farms.

The National Crime Agency is also investigating organised crime gangs from Romania, including Roma gipsy criminals who have highly organised begging and pick-pocketing rings.

Other nationalities who have been trafficked and abused include people from Poland, Slovakia and Bulgaria. Many of these people are enticed into the UK on the promise of well paying jobs and good quality accommodation, but end up being exploited.

The figures show an almost 500 per cent increase in people exploited from the Sudan.

The figures come from the National Referral Mechanism (NRM), a government safeguarding framework which authorities and charities refer potential trafficking victims to.

The North West saw notable increases in potential victims – from three to 25 on Merseyside and 30 to 89 in Manchester.

According to campaigners, migrants are duped into believing they are travelling to the UK for work and enter the country legally under the EU's Free Movement Directive.