Video report by ITV News International Affairs Editor Rageh Omaar
Bill Gates has told ITV News that Britain must stick to its overseas aid budget amid speculation Theresa May could drop the commitment from the Tory election manifesto.
The Microsoft co-founder said dropping the international foreign aid target would cost lives and insisted the cash - 0.7% of national income - bolstered UK security.
The government is under pressure from some quarters to scrap the target for overseas aid, which was put into law with the backing of the Tory-Liberal Democrat coalition in 2015 and maintained by David Cameron.
Mrs May has hinted she might drop the 0.7% commitment.
Asked if it would be included in the Tory manifesto for the June 8 snap election, the prime minister told the Sun: "You'll have to wait, and read the manifesto when it comes, won't you?"
Mr Gates, who has donated billions of his own fortune through his charitable foundation, said he wished British taxpayers could see the "incredible results" their money was delivering.
The American said giving aid is not only philanthropic, but in Britain's interest to help stabilise poorer countries.
He said the payment helps make the UK safer by reducing global outbreaks of infectious diseases or avoiding war and large-scale migration.
He said the UK's contribution makes a "huge difference" in places, for example, where conditions like polio have been almost eradicated.
Mr Gates said as the country prepares for Brexit, keeping the aid pledge signals "a sense of engagement, optimism - a belief that the UK approach, the values here can make a difference".
He also highlighted advances made by countries that have received British foreign aid, including India, Indonesia and Vietnam.
The 61-year-old admitted Africa is a "particular challenge" because of the different types of crops and agriculture, but said "the science is there, the understanding of how to get that out to people is there".
Mr Gates said agriculture productivity currently underway in drought-hit Ethiopia had "made it possible for the country to largely feed its own population".
He added: "This constant improvement in food production, education, health ... means that the risks coming in from all these other places will be greatly reduced."