Mr Gates, the world's richest man, arrives in New Delhi today to announce a $100m charitable programme to fight the virus in India.

He had been expected to meet the country's prime minister, Atal Bihari Vajpayee, but yesterday India's rightwing nationalist government made it clear that it was deeply unhappy with a study endorsed by Mr Gates that suggests 20-25 million Indians are likely to have the HIV virus by 2010.

The report by the Washington-based Centre for Strategic and International Studies , also predicted that India will overtake South Africa as the country with the largest number of Aids cases.

In an interview over the weekend India's health minister, Shatrughan Sinha, said that Mr Gates was guilty of "spreading panic". The government took a dim view of "negative" comments by the Microsoft founder and by the US ambassador in New Delhi, Robert Blackwill, who last week warned that India was on the brink of a huge HIV/Aids crisis, officials said.

Mr Vajpayee has now cancelled a meeting with Mr Gates scheduled for later today.

The unprecedented rebuff to the world's biggest philanthropist is all the more extraordinary given the fact that India's crumbling healthcare system clearly needs his money.

"I don't think anyone should contribute to spreading general panic," Mr Sinha, an actor-turned-politician, told the Indian Express newspaper.

"We are aware of the situation and it does concern me personally. But what do we achieve by saying in public that we will have 25 million people with Aids in the coming years?" he asked.

"Every year we update the information on the HIV/Aids scene in this country. And we are surprised by the figures being cited so freely."

Mr Sinha is also likely to be "unavailable" during Mr Gates' two-day trip to India, even though he recently agreed to take time out from his apparently hectic ministerial schedule to star in a Bollywood film.

India's ruling BJP party is notoriously sensitive to any criticism from abroad, and earlier this year fell out with both Britain and the European Union after they questioned its role during communal riots in the state of Gujarat.

Campaigners say the Indian government has deliberately underestimated the number of Indians with HIV/Aids, and is in "deep denial" about the sheer scale of the problem.

Since the first case of HIV in India was discovered in 1986, the disease has spread from cities via truck drivers and prostitutes into the general population. Officially, New Delhi insists only 4 million people have the virus, but other estimates claim India already has 5-8 million cases.