Inner city children being forced to sell drugs in the countryside has fueled a tripling in the number of modern slavery victims in Britain, the National Crime Agency has said.

For the first time UK nationals have made up the majority of the cases reported to the scheme set up to identify children and adults who are at the mercy of slave drivers and traffickers.

This was partly down to fears of children being exploited in a drug distribution model known as "county lines", where city gangs branch out into county or coastal towns to sell heroin and crack cocaine.

In 2017 there were 5,145 potential victims referred, up 35 per cent from 2016 and the number has almost tripled since 2013, when there were 1,745 cases.

Whilst the highest number of cases related to the exploitation of labour, there were also hundreds of reports of sexual exploitation, domestic servitude and even three cases of organ harvesting.

Of the 116 nationalities reported to have fallen victim, the largest group was UK citizens, with 819 flagged to the National Referral Mechanism, more than double the 326 referred in 2016.

NCA director Will Kerr said: "It is our assessment that the increase we are seeing here is driven by an increased awareness and greater reporting of modern slavery and that is to be welcomed.