On the first day of his six-day tour to Africa, Mr Brown put forward a four-point package to fight Aids as a central strand of Britain's presidency of the G8 this year. His prediction that a "partly effective vaccine" could be developed by 2012 will surprise some scientists, who believe a breakthrough is still a long way off.

Mr Brown may be trying to renew moral pressure on the US administration to sign up to his plan to double aid to Africa and raise an extra $50bn a year through his proposed international finance facility. Ten billion dollars of the $50bn would be set aside to fight Aids.

Mr Brown said only £40m is now being spent each year on researching and developing an Aids vaccine, despite the disease's prevalence.

"That is not nearly enough faced with complex scientific challenges," he said. "It is generally recognised that the sums of money required involve at least a doubling of money for Aids research.

"If we just keep spending at the current level, we could expect to have a partially effective vaccine for the developing world - one which could save 40 million lives -only by 2015 at best or more likely 2020, 15 years from now."

He said the doubling of R&D spending he proposes would bring forward the discovery of an Aids vaccine by three years and save a further 6 million lives.

He also proposed setting up a new international platform for research into Aids similar to the one established for the human genome project. He said he would be discussing the proposal at a seminar with the Italian treasury in London next month.

Mr Brown suggested a new way to develop a commercial market for Aids vaccines, proposing that the major industrialised countries commit themselves to buy the first 300m vaccine courses at a cost of $20 a course, so setting up a $6bn guarantee of a future market. He claimed this would be "a large enough inducement to create a much stronger interest from both large and small pharmaceutical firms". The private sector was only spending £60m a year on Aids research at present, he said.

Mr Brown's proposals seemed to be designed to sting the Americans into coming alongside his proposal for an international finance facility to frontload aid for Africa. The Bush administration has expressed deep concern about the Aids crisis but many believe it is not putting enough cash into the UN-backed Global Fund, which is fighting not only Aids but also tuberculosis and malaria. The Global Fund yesterday welcomed Mr Brown's proposals for a comprehensive global plan to combat Aids.

Speaking from Geneva, the executive director of the fund, Prof Richard Feachem, said: "Aids is not unbeatable ... But to get on top of this pandemic, we need to think big and act boldly. We need to invest up front. Unless we reach a high threshold of action and financing, our efforts are wasted. Mr Brown's ideas are big and bold - just what we need to win the war against this virus."