The Prime Minister said the new funding arrangement, announced at a global vaccine conference in central London, vaccinate more than 80 million children against diseases such as pneumonia and diarrhoea.
Britain and billionaire Bill Gates together pledged more than $2.3 billion (£1.4 billion) at an international donor conference on Monday to fund vaccination programmes to protect children in poor countries.
Speaking at the Global Alliance for Vaccines and Immunisation (Gavi) conference in London, Mr Cameron said the new funding pledge would help save 1.4 million lives in the developing world over the next five years.
The conference is being attended by world leaders, charities, private companies and philanthropists including Bill Gates, the Microsoft chief, which is discussing how to generate the funds to ensure children receive protection against potentially fatal diseases.
Gavi is facing a shortfall of £2.3 billion for its work over the next five years, charities have warned.
“Britain will play its full part,” Mr Cameron told the conference.
“In addition to our existing support for Gavi, we will provide £814 million of new funding up to 2015.
“This will help vaccinate over 80 million children and save 1.4 million lives.”
He added: “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.”
The conference will be told that the lives of millions of youngsters are at stake because of a funding crisis in attempts to keep them free from disease.
Mr Cameron and Mr Gates, one of the world’s most successful business chiefs and philanthropists, are urging the world to support the case.
Mr Gates told the conference that his Bill and Melinda Gates Foundation was pledging an additional one billion US dollars (£616 million) over the next five years to help the vaccination campaign.
The money adds to a pledge made by Australia’s Foreign Minister Kevin Rudd on Sunday who said his country would donate $210 million (£141million) to GAVI over three years until 2013.
More international donors, including the United States, France, Germany, Japan and others, are expected to add their pledges later on Monday in an effort to stump up an extra $3.7 billion (£2.27 million) needed by GAVI to fund its programmes through to 2015.
Mr Cameron acknowledged that the increased cash for vaccinations – part of the UK’s goal of devoting 0.7% of national income to aid by 2013 – would be “controversial” at a time of cuts in spending on public services at home.
He told the conference: “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.
“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 the Prime Minister rejected these arguments.
“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,” he said.
“When you make a promise to the poorest children in the world, you should keep it.”
Mr Cameron recalled watching the G8 summit at Gleneagles and the Live 8 pop concert in 2005 and thinking it was right that world leaders should make public pledges to help the poorest countries.
“For me, it is a question of values,” he said.
“This is about saving lives. 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.”
Some people were pressing him to put off aid commitments until after Britain’s economy is back on an even keel, said Mr Cameron.
But he insisted: “We can’t afford to wait. How many minutes do we wait? Three children die every minute from pneumonia alone. Waiting is not the right thing to do. I don’t think 0.7% of our gross national income is too high a price to pay for saving lives.”
And Mr Cameron said that there was a “strong practical” argument for aid, as it would help transform poor countries in Africa into trading partners for the UK, and would 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.
Mr Cameron added: “I actually think that most people in our country want Britain to stand for something in the world, to be something in the world.
“When I think about what makes me proud of our country, I think of our incredibly brave servicemen and women and of our capabilities as an economic and diplomatic power, but I also think of our sense of duty to help others.
“That says something about this country and I think it’s something we can be proud of.”
Acknowledging the problem of corruption, Mr Cameron said: “I totally get this argument. It’s right to be angry when aid is badly spent.
“Let me tell you: I’m not prepared to see a single penny of hard-earned money wasted on corrupt governments, or badly spent aid.
“But the answer isn’t to walk away from aid. It’s to change the way we do development – so that we get real results and real value for money.”
Gavi’s aim of raising £2.3 billion over the next five years will help protect at least a quarter of a billion children against killer diseases and save four million lives, said Mr Cameron.
And he asked: “In this world, where countries are tackling deficits, and more than ever before the emphasis is quite rightly on getting value for money, what greater value for money can there possibly be?”
The conference wants to save the lives of four million children and transform those of 250 million more by the middle of the decade.
The conference will bring together national governments, private companies, donors and civil society for a global push to protect the poorest in the world from preventable diseases such as pneumonia and diarrhoea.
Britain’s existing commitment is £680 million for 2011-15, but it has not been disclosed yet how much it will increase by. Britain has already vaccinated 55 million children around the world since 2000.
Earlier, Andrew Mitchell, the International Development Secretary, said Britain would show leadership at the conference.
Mr Mitchell said when the Government was elected last year there had been a “root and branch” review of all 43 of the international organisations through which Britain channels taxpayers’ money on development.
“This alliance on vaccines and immunisation came out just about top – really, really good value for money – and that is why we are giving it such strong support,” he told BBC Breakfast.
“We know that British taxpayers’ money will reach where it is really needed.
“This is our ‘big ask’ for this year. We think that this is the way that Britain can make a real difference, our taxpayers will feel the money is being really well used.”
He added: “After all, for the price of a cup of coffee you can vaccinate a child against all five of the main killer diseases which children all around the poor world … die of every 20 seconds, but children in Britain don’t die of these diseases.”
Asked about where the money would come from, he replied: “It comes out of the ring-fenced existing international development budget.”
Mr Mitchell also defended Britain’s aid budget, which despite cuts across Whitehall has increased, prompting vocal criticism from the Tory backbenches.
He said it was not only ”morally right” but in the UK’s national interests to continue to fund development projects around the world.
He said: ”We don’t protect our security only by tanks and guns but also by training the police in Afghanistan, getting girls into school in the Horn of Africa and building up government structures in the Middle East.”
Mr Gates was in optimistic mood ahead of the conference, saying he thought it would be ”very, very positive”.
In an open telephone conversation, hosted by broadcaster and Save the Children Ambassador Natasha Kaplinksy, Mr Gates said a malaria vaccine could be just a few years away, while polio could follow smallpox in being eradicated thanks to the success of its vaccine.
Vaccines were ”magic”, he said.
”They are very inexpensive, they can protect you for your entire life, so diseases like smallpox that used to kill millions are completely gone because of the vaccine. It’s the greatest thing that ever happened in human health. We need to get them out to people and invent some more.”
Gavi’s global immunisation programme includes the roll out of new vaccines against two of the biggest child killers, pneumonia and diarrhoea.
Save the Children’s new report, Vaccines for All, reveals that there are still around 24 million children – the world’s poorest, and those most vulnerable to disease – that have no access to vaccines.
This could be changed if Gavi gets the resources it needs, it says.
Jemima Khan, who is an ambassador for Unicef, speaking on the same programme, praised the Government for “taking the lead” on the issue.
“This conference is about saving four million extra lives by 2015. At the moment, one child every 20 seconds dies of a vaccine-preventable disease. That is still a very shocking figure given that we have the know-how, we have the expertise to do something about that,” she said.
Harriet Harman, the Shadow international development secretary, said it was “unacceptable” that millions of children in the developing world die from illnesses which could be prevented by vaccinations which are taken for granted in the UK.
“Today, there must be bold contributions from donors to ensure that Gavi has the resources it needs to save millions of children’s lives and that the progress that has already been made does not stall,” said Miss Harman.
“The private sector must also play its part by supplying vaccines at the lowest possible prices.”