Human traffickers smuggling children from northern France to Britain could slip under the radar if law enforcers stop cooperating when Britain leaves the European Union, charities said yesterday.

Many children are trafficked to Britain from France's northern port of Calais, where asylum seekers and economic migrants still gather, despite the dismantling of a huge camp known as 'The Jungle' in late 2016, rights groups say.

Britain must ensure its cross-border partnerships with France to protect children and tackle trafficking are preserved after Brexit next March, said the National Society for the Prevention of Cruelty to Children (NSPCC).

Europe's policing and judicial agencies, Europol and Eurojust, and the European Arrest Warrant, are vital to Britain's efforts to find child victims and prosecute traffickers, said Almudena Lara, head of policy at the NSPCC.

"Britain has unfinished business with protecting the most vulnerable children from trafficking in Europe, and Brexit threatens to further weaken its response to this issue," said Jakub Sobik, a spokesman for charity Anti-Slavery International.

Joint investigations between British and European police forces could be jeopardized if Britain loses funding from Europe, said Tamara Barnett of the Human Trafficking Foundation.

"The fear is that once we [Britain] leave [the European Union], our forces might not receive funding for this type of work, particularly since police resources are increasingly being cut anyway," Barnett told the Thomson Reuters Foundation.

The Home Office said the support Britain offers to trafficked children would not be affected by Brexit.

"We are committed to strong cooperation on security, law enforcement and criminal justice now and after we leave [the European Union]," the spokesman said in a statement.

Of nearly 200 children living in camps in northern France referred to the NSPCC between August 2016 and November last year, about a third ended up in Britain. The whereabouts of the others remain unknown, according to a report by the charity.

Many of the children were sexually abused, made to sleep in rat-infested tents and beaten by traffickers, the report said, after many of them had fled conflict and crisis further south.

Britain's exit from the European Union could hamper its fight against trafficking at a time when the number of people being trapped in slavery is on the rise, fueled by social media, the country's anti-slavery body said earlier this month.

The Gangmasters and Laor Abuse Authority (GLAA) said it was unclear how Brexit would affect Britain's drive to crack down on human traffickers, but it was the key factor likely to impact the intelligence picture in coming years.

Britain passed the Modern Slavery Act in 2015 to crack down on traffickers, ensure businesses check supply chains for forced labor, and protect people most at risk of being enslaved.

In Britain, at least 13,000 people are estimated by the government to be victims of modern-day slavery - used in forced labor, sexual exploitation or domestic servitude - but police say the true figure is likely to be in the tens of thousands.