Bill Gates gives 5 billion dollars a year to development aid, making him one of the world's most generous philanthropists.
In a speech at London's Royal United Services Institute this week,
he voiced fears that the political tide is turning against foreign aid.
"It concerns me that some world leaders are interpreting recent events as reasons
to turn inward instead of seeing them for what they are:
problems that although they are difficult and will take time, can be solved—
if we invest in the long-term solutions that are necessary".
The United States remains by far the world's biggest donor, funding long-term programs and emergency relief across the globe
such as this 1.8 million-dollar aid package sent to Peru following floods earlier this year.
But U.S. President Donald Trump is proposing to cut the 43 billion-dollar foreign aid budget
as part of efforts to reduce government debt.
Gates argues many critics of foreign aid don't realize the huge progress that has been achieved.
"If you could only pick one number to highlight the effectiveness of the development agenda since 1990,
I would pick the number 122 million, that's the number of children's lives that have been saved".
He disputed the notion that funding foreign aid is a bottomless pit.
"As you bring down that childhood death rate, families choose to have less children.
The population goes down very substantially, which brings within reach all of the things society is trying to do:
better health, better education, economic opportunity."
Bill Gates' speech in London comes as Britain gears up for a snap election in June.
The UK is one of the few developed countries to meet the U.N. aid budget target of 0.7 percent of GDP.
Current Prime Minister Theresa May has committed to keeping that pledge
but many in her party want aid money diverted to the military.
Gates said he wanted to make the case for the facts.
"When aid is mismanaged, it is a double crime, stealing both from the taxpayer and the poor.
But let's be clear, the bulk of this aid is getting to its recipients and having an incredible effect.
There will always be a need to adjust, we're working in very tough countries, and so you'll never get 100 percent perfect effectiveness.
But you can learn, and every year, the aid is better spent".
Aid agencies say the debate couldn't come at a worse time, with around 65 million refugees around the world,
worsening conflicts in the Middle East and famine striking East Africa.
Henry Ridgwell, for VOA News, London.