David Cameron today 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 Prime Minister acknowledged that his Government's pledge to maintain international aid spending was "controversial" but said developed nations must keep their promises to the world's poorest.
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.
"That is one child vaccinated every two seconds for five years. It is one child's life saved every two minutes. That is what the money that the British taxpayer is putting in will give," he said.
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."
Recalling the G8 summit at Gleneagles and the Live 8 pop concert in 2005, he added: "It was the right thing to promise. It was the right thing for Britain to do. And it is the right thing for this Government to honour that commitment."
He also pointed to a "strong practical" argument for aid, saying it would help transform poor countries in Africa into trading partners for the UK, and prevent the conditions which lead to mass migration, radicalisation and terrorism.
"If we invest in countries before they get broken, we might not end up spending so much on dealing with the problems. Whether that's immigration or new threats to our national security," he said.
In total, today's 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.
Its total resources between now and 2015 will be 7.6 billion dollars (£4.6bn), of which 2.4 billion (£1.5bn) will come from Britain. Its aim is to immunise more than 250 million of the world's poorest children against life-threatening diseases by 2015.
Mr Gates said: "For the first time in history, children in developing countries will receive the same vaccines against diarrhoea and pneumonia as children in rich countries.
"Together we must do more to ensure that all children - no matter where they live - have equal access to life saving vaccines."
Justin Forsyth, chief executive of Save the Children, hailed a "huge boost" to the world's poorest children as a result of today's pledges.
"Millions will now survive common childhood illnesses like pneumonia and diarrhoea, enabling them to grow up and fulfil their potential," he said.
"UK leadership has been critical to this success, but as David Cameron said today, promises to the world's poorest have to be kept.
"The hard work begins now to keep these historic commitments on track and make sure the vaccines get to the children who need them most."
Oxfam senior policy adviser Max Lawson said: "David Cameron deserves real credit for tackling the aid critics head-on and committing much-needed funds to vaccinate children in poor countries.
"The Prime Minister is right to say that vaccination is one of the most effective ways of improving health in poor countries."
International Development Secretary Andrew Mitchell said support for immunisation and vaccination through Gavi would ensure that taxpayers got value for money in aid spending.
"We made it clear that we would not balance the books on the backs of the poorest people on the planet. I have no doubt that was exactly the right decision," he said.
"Of course having ringfenced that budget and made that decision we owe it to our hard-pressed taxpayers to ensure that we get a pound of development value for every 100 pence hard-pressed taxpayers produce.
"That is why we made tough decisions to take money away from some organisations but in terms of vaccination and immunisation - the cost-effectiveness of it, the value for money of it, the fact that it is undoubtedly the right thing to - I think the British taxpayer can have confidence this money is being really well spent."