The National Crime Agency (NCA) said it received 5,145 reports of suspected slavery victims in 2017 across Britain, an increase of more than a third from 3,804 in 2016.
Those involved were commonly British, Albanian and Vietnamese, said the NCA, which is dubbed Britain's FBI, warning that criminal networks were heading online to sexually exploit people, especially through adult services websites.
"What this report reinforces is that we are now dealing with an evolving threat," NCA director Will Kerr said in a statement. "Particularly concerning to us is the rise in young people being exploited for sexual purposes or drug trafficking."
The NCA said the jump in numbers was largely due to British children being reported to authorities as suspected victims of sexual exploitation or being used by gangs to carry illegal drugs in the so-called 'county lines' drug trade.
There were also increased crossovers between smuggling rings, that transport vulnerable migrants into Britain, and slavemasters, who force them into exploitative work, Kerr said.
Kerr said the figures "almost certainly" represent an underestimate of the true scale of slavery and trafficking in Britain.
Britain is regarded as a leader in global efforts to combat slavery, with its 2015 Modern Slavery Act introducing life sentences for traffickers, forcing companies to address the risk of forced labour, and better protect people at risk.
The National Police Chiefs' Council (NPCC) said on Monday it held 568 police operations in February alone but despite more arrests, authorities are struggling to jail slavemasters.
Trafficking prosecutions rose to 295 in 2015/2016 from 187 in 2014/15, but have since levelled off, according to data from the Crown Prosecution Service (CPS).
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)
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