UK Prime Minister David Cameron has pledged £814m to help vaccinate children around the world against preventable diseases like pneumonia.He made the announcement at a summit in London where countries were being asked to give an extra £2.3bn ($3.7bn) by 2015 for child vaccines.
Donor countries have agreed to commit more than this - $4.3bn.
The Global Alliance on Vaccines and Immunisation says this could help save four million lives in four years.
The UK has already given more, £2bn over 30 years, than any other nation towards immunising some of the world's poorest children against life-threatening diseases.
The extra £814m ($1.3bn) comes on top of the UK's existing commitment of £680m between 2011 and 2015.
Hosting the conference alongside Mr Cameron, Microsoft tycoon Bill Gates said he would be giving $1bn to help the campaign. 'Expertise'
Mr Cameron said there was a "strong moral case" for keeping pledges Britain had made to the developing world, no matter the economic circumstances at home."Today we come together because we have the chance to save another four million lives."
He said the idea of children dying from pneumonia and diarrhoea should be "unthinkable" in 2011.
"To those who say fine but we should put off seeing through those promises to another day because right now we can't afford to help: I say - we can't afford to wait."
Two million under-fives die from pneumonia alone each year despite the existence of a vaccine to protect them.
It is estimated that three times as many children aged under five die from pneumonia and diarrhoea than from malaria and HIV/Aids combined, despite new vaccines being available to help prevent such deaths. However, many developing countries cannot afford them.
'Make-or-break' Drugs company GlaxoSmithKline last week agreed to sell a vaccine for diarrhoeal disease at cost price to poorer nations, and some other firms have since made similar moves.
GAVI has already rolled out a range of vaccines to children in 72 countries but the organisation says it needs the extra money to vaccinate even more children against pneumococcal disease and rotavirus.
Resources will also be spent on trying to reach millions of the poorest children who are missing out on basic vaccines against diseases such as measles, whooping cough and tetanus.
GAVI is a health partnership of governments, businesses, and bodies including the World Bank, Unicef and Mr Gates' personal foundation.
The philanthropist Mr Gates said: "I just wanna thank everybody for this incredible milestone. Four hours is a long time but if you can save four million lives in four hours it's well worth every minute.
"For the first time in history, children in developing countries will receive the same vaccines against diarrhea and pneumonia as children in rich countries."