Twenty years after the explosion at the nuclear reactor at Chernobyl in the Ukraine, the legal fallout has just reached France.

Professor Pierre Pellerin, who was the head of France's nuclear safety watchdog 20 years ago, has been formally accused of deliberately concealing the seriousness of contamination of parts of the French countryside from the French people.

An investigation is continuing into the responsibilities of politicians in the alleged cover-up, including the role of Jacques Chirac, who was the prime minister. But for the time being, anti-nuclear campaigners and a group of 500 thyroid cancer sufferers are celebrating a first victory in a marathon legal campaign.

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Professor Pellerin, now 82, has been placed under formal investigation for "aggravated deception", but a potentially more serious accusation of causing "involuntary bodily harm" was dropped on Wednesday.

At the time of the explosion at the Chernobyl reactor on 26 April 1986, the professor was head of the agency, attached to the Health Ministry, which reported on risks to health. As the "cloud" of contamination passed over France between 30 April and 5 May that year, Professor Pellerin issued a series of reassuring statements. He published low average findings of radiation across whole regions. Campaigners have long protested that this deliberately concealed the fact that there were pockets of contamination which suffered high rainfall as the Chernobyl cloud moved westwards.

In a four-hour interrogation by a judge on Wednesday, Professor Pellerin said that he had issued accurate and balanced information to the public. The investigation is likely to continue for several years and the case may never come to trial.