"While people around the planet have been enjoying and competing in these Games, there's another world where children don't have enough to eat and never get the start in life they deserve," he said.
"We've a responsibility to tackle this. But the hard truth is that, while we've made huge strides in the last decade on things like education, malnutrition rates have stagnated."
He has also set aside the 250,000 pounds ($389,000) he won in a TV show earlier this year for the project -- and there is more to come.
"On the first of September I'm having a big charity auction for the Mo Farah Foundation," he said on Saturday.
Olympians including Ethiopian 10,000 metres gold medallist Tirunesh Dibaba and British long jump winner Greg Rutherford have signed an open letter urging Cameron to "fire the starting gun on the biggest ever push against hunger and malnutrition".
Cameron was set to pledge £120 million ($188 million, 153 million euros) for research into drought-resistant and vitamin-enhanced crops for Africa and South Asia.
The conference comes ahead of the closing ceremony of the London Olympics on Sunday, which will see the Olympic flame extinguished in the British capital and organisers hand over to the Rio 2016 team.