An investigation has begun in France after unidentified drones were spotted flying over seven nuclear plants.

State-owned power company EDF said most of the flights took place between 13 and 20 October either at night or early in the morning.

Yannick Rousselet from pressure group Greenpeace said the security lapse was concerning: “To be able to enter the airspace of French nuclear power plants or French nuclear installations – to enter, and to leave – without anybody intervening. I think there are true flaws to the security of our facilities.”

Suspicion had initially fallen on Greenpeace as its member have previously sneaked into nuclear sites to highlight security lapses. However the organisation said it is always very open with its activities and had nothing to do with the drones.

France’s Interior Minister Bernard Cazeneuve said measures are being taken to “neutralise” the drones.