Save the Children is urging David Cameron and other world leaders to help fund the £2.3 billion cost of immunising the world's poorest children over the next four years.
The Global Alliance for Vaccines and Immunisation (GAVI) would use the money to immunise 243 million children by 2015 and save four million more lives.
Nearly two million children die from vaccine-preventable diseases each year.
It is a "make-or-break" situation, says the charity.
GAVI leads worldwide efforts to improve access to vaccines, bringing together governments, international organisations and pharmaceutical companies to achieve its aims.
Since 2001, it has enabled 288 million children to be vaccinated and has already averted five million deaths.
But to introduce new vaccines in the poorest countries of the world, GAVI requires £4.2 billion, of which £1.9 billion is already pledged.
In a report called 'Vaccines for All', Save the Children is calling on developed countries to pledge donations at a conference in London on Monday 13 June.
Justin Forsyth, the charity's chief executive, said that everyone has a part to play.
"World leaders have to find the funds, the private sector has to supply the vaccines at special discount prices, and developing world governments have to prioritise the delivery of vaccines, through their national health services, to help millions more children survive."
Vaccines already save the lives of around 2.5 million children every year but Save the Children says they have the potential to save many, many more.
Statistics from Sierra Leone show that a child who receives all his or her basic immunisations is at least six times more likely to survive than a child with none.
Yet one-fifth of the world's children - around 24 million - still do not receive any life-saving vaccines.
Most of the world's unvaccinated children live in its poorest countries.
Chad has the highest percentage of unvaccinated children, at around 77%. Somalia, Equatorial Guinea and Nigeria are not far behind.
India has over nine million unvaccinated children - more than any other country.
Save the Children says donors must commit enough money "to ensure that the true potential of vaccines is realised".
Some of GAVI's more recent activity has involved introducing new vaccines against major causes of pneumonia and diarrhoea - the two of the biggest killers of children under five.
But the alliance's work also includes distributing and administering the vaccines, which means arranging for nurses and doctors to be trained, supported and paid.
Save the Children is also calling on vaccine manufacturers to work together to reduce the prices of new and existing vaccines.
Earlier this week drugs company GlaxoSmithKline (GSK) said it would cut the price of its vaccine for rotavirus by 67% to $2.50 (£1.50) a dose in poor countries.
Several other major drugs companies have announced big cuts to the amounts they charge for their vaccines in the developing world.
"That way, donors can buy more vaccines and poor countries can afford them long-term," the charity says.
The UK government is currently GAVI's largest government donor, contributing $360 million dollars over 20 years.