Mr Brown said vaccinating now was 'the right thing to do'

The bond scheme effectively allows cash due to be spent on vaccinations over the next decade to be used now.

Mr Brown said he hoped it would fund 500 million immunisations in 10 years.

The chancellor, who is to be joined by religious leaders at the launch, said: "There is no better case for advancing money than vaccination."

The diseases targeted will include tuberculosis, diphtheria and tetanus.

'Need money now'

Under the international finance facility scheme, revenue from bonds will become available earlier than would have be the case with normal aid budgets.

Mr Brown told the BBC: "We want to spend the money now. If you can vaccinate 100% of children, then we avoid the risk of disease and contagion.

"If you can only vaccinate 10% of the children one year and 10% the next year and 10% in future years then you have still got a huge risk of disease."

Mr Brown said he hoped to persuade countries including the US, Brazil, China and South Africa to join the scheme, which sees the bonds sold on behalf of the World Bank.

He suggested that the US administration did not have the "political will" to commit itself to this form of aid over the long term.

The bulk of the money will come from the UK aid budget and Microsoft founder Bill Gates' charity.

Mr Brown denied that he had opted for this complicated form of funding to keep additional aid spending off the government's books.

Conservative international development spokesman Andrew Mitchell welcomed the scheme, but said "we must keep a very close eye on exactly how these funds are spent, and how effective they are".

Pope Benedict XVI's envoy, Cardinal Renato Raffaele, and the Archbishop of Canterbury, Dr Rowan Williams, are supporting the International Finance Facility for Immunisation.

It also has the backing of Chief Rabbi Dr Jonathan Sacks, Iqbal Khan of the Muslim Forum, Ramesh Khalidi of the Hindu Forum, and Indrajit Singh of the Sikh Council of Britain.