The Paris prosecutor's office ordered an investigation in July after a group of 51 plaintiffs with thyroid ailments filed a suit against the government for involuntary physical injury and wilful disregard of duty to ensure public safety.

They alleged French authorities failed to warn the public of the health risks after a radioactive cloud drifted west from Chernobyl in Ukraine when a reactor exploded in April 1986.

Under French law, a probe is one step short of charges.

A lawyer for the new plaintiffs said they were mostly from Paris, eastern France and Corsica, the regions worst affected.

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 connection with Chernobyl.