Brexit ‘poses an existential threat to the UK’s anti-slavery efforts’

She will then head to Lagos to announce business projects, develop financial links amid post-Brexit uncertainty for the City of London, and meet survivors of modern slavery.

UK and French support to help Nigeria and Niger develop their borders to prevent trafficking, and counselling plus training to help 1,700 migrants and victims of modern slavery are among the pledges to be made.

But Aidan McQuade, a former CEO of of Anti-Slavery International, today warned Mrs May that Brexit would have a severe impact on efforts to tackle the problem.

Mr McQuade, a supporter of the anti-Brexit campaign group Best for Britain, said: “The prime minister must recognise that Brexit poses an existential threat to the UK’s anti-slavery efforts.

“These are factors that provide opportunities for unscrupulous employers to exploit and enslave vulnerable workers.

“Co-operation with European countries is essential to tackle modern slavery. Mr McQuade said he believed it was difficult to see how the UK would ever be able to ensure anti-slavery provisions in future trade deals with states where slavery is endemic.

“In negotiations with such states the UK may well be reduced to the role of an abject supplicant in no position to make human rights demands, such will be the level of its desperation to replace the trade Brexit will lose it with Europe,” he said.

“That’s why we need to stay in the EU and have a people’s vote on the final Brexit deal.”

The prime minister arrives in Nigeria after making her pitch for the UK to help develop Africa, particularly in the post-Brexit world, and seize a greater share in the market amid fierce global competition from China, France and the United States.

She used a visit to South Africa to also outline her plan for how overseas aid should work in Britain’s interests and announced Britain’s first post-Brexit trade pact with a group of southern African countries.

But with Africa representing a small percentage of the UK’s trade, more will have to emerge beyond the agreements announced to satisfy Brexit critics who believe it will be difficult to replicate the advantages of EU membership.

A lighter moment in her first stop in South Africa saw May dance with schoolchildren before jokingly playing down her chances of appearing on Strictly Come Dancing.

Ahead of her trip to Nigeria, May described modern slavery as “one of the greatest human rights abuses of our time” and labelled the UK a “world leader” in trying to end the crime.

She said: “Today we are stepping up our partnership with Nigerian authorities to find traffickers and bring them to justice.”

On business matters, she added: “London is a world-leading financial centre and, as the UK leaves the European Union, it will play an even greater role in financing the fastest-growing economies across Africa and the world.”

May’s three-day trade mission ends in Kenya tomorrow.

She has been joined by junior ministers and business leaders.