David Cameron and other world leaders have a make-or-break opportunity to help save the lives of four million children in just four hours when he leads a global vaccine summit in London on Monday, according to Save the Children.

The charity is urging the Prime Minister and other world leaders to fill the immunisation funding gap at the half-day conference to ensure the world's poorest children receive life-saving vaccines for major leading childhood illnesses like pneumonia and diarrhoea.

The Global Alliance for Vaccines and Immunisation is facing a shortfall of £2.3 billion for its work over the next five years. Without full funding, many children - nearly a quarter of the global child death toll - will continue to die from vaccine-preventable diseases every year.

­Justin Forsyth, Save the Children's Chief Executive, said: "We are on the cusp of an historic breakthrough that would save millions of lives. We have new vaccines to tackle appalling common childhood killers like diarrhoea and more children than ever are getting immunised against illnesses like whooping cough. But without pledges on June 13th, that incredible progress could stall."

In a new report, entitled "Vaccines for All", Save the Children said with new funds available, more poor children currently missing out on immunisations taken for granted in the UK, will be saved. It also called on vaccine manufacturers to provide the vaccines at the lowest possible prices, to ensure the maximum number of children is helped.

Justin Forsyth said, "Everyone has to play their part. 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."

Save the Children's report says that immunising children is arguably the greatest health success story of the last century.