Britain and Bill Gates joined forces on Monday to pledge more than half the 3.7 billion dollars (2.6 billion euros) sought to vaccinate millions of children by 2015.
Prime Minister David Cameron told a donors' conference in London that Britain would contribute a further Ģ814 million (1.3 billion dollars, 920 million euros) to GAVI (Global Alliance for Vaccines and Immunisation), an aid group backed by the Microsoft tycoon.
He said the extra funds would treat 80 million children against preventable conditions such as pneumonia and diarrhoea, and save 1.4 million lives.
Gates followed Cameron's pledge by saying his charitable foundation would commit an additional $1 billion over five years.
Cameron said: "Today we come together because we have the chance to save another four million lives in four years.
"Frankly the idea of children dying from pneumonia and diarrhoea should be absolutely unthinkable in 2011.
"But for many parents in the developing world it is a devastating reality."
Cameron said GAVI was a "great organisation" that had been one of the top performers in a British government review of aid groups.
The pledges have gone a long way towards GAVI's aim of making up the $3.7-billion funding shortfall in its aim to immunise 243 million more children and avert four million deaths by 2015.
GAVI has already vaccinated 288 million children in 19 countries.
It wants to extend the vaccination programme to another 26 countries.
Announcing his pledge, Gates said: "It's not every day you give away a billion dollars but for a cause like this it's exciting to be doing this."
Pneumonia and diarrhoea kill three times as many children under the age of five as HIV/AIDS even though vaccines are available to prevent such deaths.
Many developing countries cannot afford the vaccines.
British drug giant GlaxoSmithKline last week agreed that for sales to the world's poorest nations it would slash 95 percent off the price of a vaccine for the diarrhoeal disease rotavirus.
Cameron told the conference Britain's increased contribution to GAVI was in line with his decision to maintain the government's international aid budget at a time when it was making cuts in many other areas.
But he said he wanted to see poor nations become trading partners, not just recipients of aid.
"In the long term I know we will not help countries develop by just giving them money.