Britain must be “unashamed” about using its £13.1bn aid budget to promote “our national interest” by boosting free trade with the developing world post-Brexit, Theresa May will say on Tuesday in a keynote speech due to be delivered in South Africa.

Addressing a delegation in Cape Town on the first day of her visit to Africa, the Prime Minister will signal a major shift in the way that aid is spent so that it provides a platform for British businesses to invest in emerging markets.

Mrs May, who landed in Cape Town on Tuesday, will state her desire to transform foreign aid so that it is “fully aligned” with Britain’s national security priorities, helping to stabilise countries in crisis and prevent…