Yuri Bandazhevsky is a 47-year-old specialist in the effects of nuclear radiation, considered by the human rights group Amnesty International to be a prisoner of conscience.

Nearly 90 percent of Belarus, north of Ukraine where the Chernobyl explosion happened, was affected by the radiation, and Bandazhevsky criticised authorities in the hardline former Soviet republic of Belarus for their management of the aftermath.

Bandazhevsky's wife Galina told AFP he had been rushed to a Minsk hospital and at first diagnosed as having an ulcer, gastritis and hepatitis,.

But doctors had then ordered an alimentary canal check and said they would establish an official diagnosis later, his wife added, insisting Bandazhevsky did not suffer from hepatitis.

Bandazhevsky was arrested in 1999 and charged with corruption and accepting bribes. He was convicted in 2001 and sentenced to eight years in a hard labour camp, later reduced to six.

Amnesty International believes he was arrested as a result of his research into the Chernobyl nuclear disaster in 1986.

When the reactor in the Chernobyl nuclear power station exploded in April 1986 it released a huge cloud of radioactivity over a large part of Europe.

Nearly six million people continue to live in contaminated zones, according to UN figures.

Bandazhevsky's case has been taken up by Amnesty International, CRIIRAD (Commission of Independent Research and Information on Radioactivity), Greenpeace, Friends of the Earth and other groups which have denounced Belarus' legal process as "iniquitous" and violating rule of law.

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