Jiang Yanyong is one of several potential critics thought to have been taken from Beijing or put under house arrest ahead of the 4 June anniversary.

Rights groups have called on China to review its handing of the protests.

Hundreds of people were killed when troops and armed police opened fire on unarmed protesters around the square.

Jiang Yanyong was the doctor who first shed light on a government cover-up of the Sars virus by contradicting official figures for the spread of the deadly disease.

He caused further embarrassment when he wrote a letter to the country's top leaders in February 2004, asking them to admit mistakes in the handling of the 1989 Tiananmen protests.

Jiang and his wife have not been seen in their Beijing home since early on 1 June, according to their daughter Jiang Rui, who is demanding an investigation.

"While we do not want to speculate as to what happened to our parents, we believe the authorities of Beijing 301 Military Hospital [the hospital where Dr Jiang works] are deliberately withholding information from us," she told Reuters news agency.

Jiang and his wife are not the only people to have gone missing in the run-up to the anniversary.

Another prominent activist, Liu Xiaobo, could no longer be contacted and may also have been taken out of the capital, according to the Hong Kong-based Information Centre for Human Rights and Democracy.

Others have been reported missing, and phone lines of known activists have been cut.

Universities are being monitored to prevent commemorations taking place.