David Cameron has committed an extra £814 million of British money to the vaccination of children in poor countries, insisting the UK had a "moral" obligation to help despite public spending cuts at home.
The announcement came as world leaders, charities, companies and billionaire philanthropist Bill Gates gathered in London for a fund-raising conference by the Global Alliance for Vaccines and Immunisation (Gavi).
The extra money will more than double the UK's contribution to the initiative, previously set at £680 million between 2011 and 2015.
Britain is by far the biggest source of funding for Gavi, followed by the Bill and Melinda Gates Foundation, which has promised to provide 1.3 billion US dollars (£800m).
Mr Cameron said the UK's money would help vaccinate more than 80 million children, saving 1.4 million lives.
But, mindful of criticism of the Government's commitment to protecting aid spending at 0.7% of national income, he also struck a defensive note.
"At a time when we are making spending cuts at home what we are doing today and the way we are protecting our aid budget is controversial," the Prime Minister said. "Some people say we simply can't afford spending money on overseas aid right now, that we should get our own house in order before worrying about other people's problems.
"Others see the point of helping other countries to develop, but they don't think aid works anyway, because corrupt dictators prevent it from reaching the people who really need it."
But went on: "I think there is a strong moral case for keeping our promises to the world's poorest and helping them, even when we face challenges at home. When you make a promise to the poorest children in the world, you should keep it."
In total, the conference secured funding commitments of 4.3 billion dollars - including an extra one billion from Microsoft founder Mr Gates - exceeding a target in advance of 3.7 billion dollars.
