The decision follows legal moves begun by a group of 51 plaintiffs with thyroid ailments who allege French authorities failed to warn the public of the dangers of radioactive fallout from the world's worst nuclear disaster. The sources said the prosecutor's office had determined there were sufficient grounds to launch an inquiry into the complaint, which the group filed against persons unknown for unintentional injury and associated counts. 

An investigating magistrate will conduct the probe. Such a move under French law does not necessarily lead to charges. The plaintiffs, backed by two pressure groups, allege the French authorities did nothing to alert people to the potential dangers from a radioactive cloud that drifted west from Chernobyl when a reactor exploded in April 1986. The plant in Ukraine shut down for good last December. 

Last year, a 31-year-old Frenchman suffering from thyroid cancer, Yohann van Waeyenberghe, lost an attempt to have criminal proceedings launched against French officials for alleged bodily harm in the Chernobyl affair. A court ruled Waeyenberghe could not demonstrate a scientific link between his illness and the accident. 

Radioactivity from the Chernobyl explosion drifted across France between April 27 and May 5, 1996. West Germany, Austria and Italy took various precautions, including restrictions on the consumption of milk and dairy products, but French authorities said there was no need for special measures to protect against any health risks. 

An official French scientific study published last December estimated the incidence of thyroid cancer in France had risen fivefold among men and more than doubled among women between 1975 and 1995. 

The study, however, said the rise had been noted before the Chernobyl disaster and that the causes had not been established. It criticised the authorities for failing to monitor the population for evidence of cancer risks after the accident.