Britain will almost double the amount of money it will pledge to a multimillion pound fund to fight Aids, and try to shame the US into raising its contribution.

Clare Short, the Secretary of State for International Development, said she had set aside £125m for the Global Health Fund to match the $200m (£140m) pledged by President George Bush. She plans to confirm the gesture at next week's special UN assembly on HIV/Aids in New York.

"I had set aside £75m but I'm upping that to £125m," she said. "But I won't commit it yet because I want to work with other countries.

"When I met the US administration in the spring they said they were proposing a substantial commitment. But for such a big country one might hope they would be trying to add to that."

The World Bank has urged the industrialised West to "respond generously". Its president, Jim Wolfensohn, said: "Rich countries must ensure their pledges of financial support represent genuinely new money. I hope and trust that they will contribute significantly to the war-chest that will be needed."

The UK, the US and France are the only countries to have pledged cash. The founder of Microsoft, Bill Gates, has pledged $100m (£67m) from his private foundation.

Britain was instrumental in persuading the US Treasury Secretary, Paul O'Neill, to back the idea of a global fund at the spring meeting of the Group of Seven (G7) richest nations.

The fund will buy drugs from the giant pharmaceutical companies to tackle Aids, as well as malaria and tuberculosis, in exchange for an agreement by the private sector to cut prices, organise distribution of the drugs and perform research into new medicines. "If the world gets together the prices [of drugs] come down because there's a great reliability of supply," Ms Short said.

She added that it was vital that the fund was part of a full-scale reform of health care in the developing world that would include distribution networks and new basic health systems. "Even if the drugs were free, people would not get them now because there's no delivery system," she said.

GlaxoSmithKline, the UK company that is the world's largest supplier of HIV medicines, has already pledged to cut its prices to 63 countries in the face of international pressure. Coca-Cola has pledged its marketing network in Africa to the effort.

Fifteen million people have died and 34 million are HIV-infected, including 25 million in sub-Saharan Africa. On top of the 3 million who die of Aids every year, 1.7 million more are struck down by TB, and 1.2 million die from malaria. But the initiative has been criticised by non-government organisations (NGOs) which believe the fund will simply subsidise the profits of the drugs multinationals.

Mark Curtis, head of policy at the charity Christian Aid, said: "There is a serious risk it will divert attention and money away from reducing the underlying poverty on which HIV/Aids thrives."

But Ms Short said blanket rejection of the fund was "not constructive and wrong. NGOs have to take extreme positions to get their names in the newspapers".

Ms Short criticised Kofi Annan, the UN secretary-general, for saying that between $7bn and $10bn would be needed to operate the fund.

"Talk of $7bn or $10bn is unwise," she said. "We need improvement in health spending but not through one fund. If you focus on a number it will be seen as a failure because the number is never big enough."

A management structure is expected to get approval at the UN session and the issue will go the G7 meeting next month in Italy.

"The G7 will support it and the technical details will be finalised so that it could be up and running by the end of the year. I think that's achievable," she said.

Ms Short was praised by Gordon Brown, the Chancellor of the Exchequer, for her role in getting the G7, which includes Canada, France, Germany, Italy and Japan, to agree in March to contribute to the fund.