By Holly Williams BBC, Beijing

China's most prominent Aids campaigner has disappeared and may be in custody.

Wan Yanhai is an outspoken advocate for China's Aids sufferers and a critic of the government's slow response to the spread of HIV.

In July, authorities banned the activist group he helped found, the Aids Action Project, but Mr Wan continued his campaigning.

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Now friends and relatives have not seen him for several days.

Mr Wan has been a long-term irritant for the Chinese government, and while the authorities here have not confirmed that he has been detained, his friends told the BBC they feared the worst.

Mr Wan's most recent campaign has been to publicise the plight of the thousands of Chinese infected with HIV by illegal blood dealers.

For several years, Chinese authorities permitted the insanitary collection of blood that infected whole villages in central China with the HIV virus.

The government's involvement has caused a scandal.

Mr Wan's website has documented the spread of the disease through the blood dealing and published a list of those who have already died.

That has drawn the ire of Chinese authorities. They banned Mr Wan's Aids Action Project in July.

When the BBC visited the group's premises, Mr Wan's colleagues appeared to be under close police surveillance.

They told the BBC that Mr Wan's disappearance is directly related to his criticism of the government's involvement in the spread of Aids.

China now has 1.5 million HIV carriers, and doctors warn that could climb as high as 10 million by 2010.

The United Nations has criticised China's response to the virus, and warned that the country faces an Aids catastrophe.