British Prime Minister Boris Johnson arrives for a meeting at Stormont Castle on the Stormont Estate in Belfast, Northern Ireland on Monday. Johnson hailed a "historic time" for Northern Ireland ahead of a visit to mark the reopening of its power-sharing government after three years of deadlock. Photo: AFP

UK Prime Minister Boris Johnson told African leaders on Monday that Britain would be more open to migrants from their continent after Brexit as he hosted a summit intended to boost trading ties.

He also promised an end to direct UK state investment in thermal coal mining or coal power plants overseas, saying London would focus on supporting a switch to low-carbon energy sources.

Johnson was speaking at the start of the first UK-Africa ­Investment Summit in London, a clear pitch for business less than two weeks before Britain leaves the European Union.

He told leaders including presidents Abdel Fattah al-Sisi of Egypt and Uhuru Kenyatta of Kenya that he wanted to make Britain their "investment partner of choice."

After highlighting all that Britain has to offer, he said Brexit would mean an end to preferential treatment for EU migrants.

"Our [immigration] system is becoming fairer and more equal between all our global friends and partners, treating people the same, wherever they come from," he said.

"By putting people before passports we will be able to attract the best talent from around the world, wherever they may be," Johnson said.

Nigerian President Muhammadu Buhari, who also attended the summit, said Brexit offered an opportunity for increased free trade across the Commonwealth - and said visas were a key issue.

The British government's export agency reports providing 2 billion pounds ($2.6 billion) in financing for UK company exports to Africa in the past two years. The agency says it now wants to "increase its risk appetite" in Egypt and the emerging economies in Nigeria and Rwanda.

The UK government said the London summit will see British and African firms announce commercial deals worth 6.5 billion pounds.

Britain will leave the EU on January 31, although ties will remain the same for 11 months while the two sides thrash out a new trading relationship.

The UK has said it will be leaving the bloc's single market and customs union.

Johnson wants the freedom to strike trade deals with other countries, even at the expense of some of its producers facing trade tariffs and quotas as a result.