A French court has fined energy giant EDF 1.5 million euros and sent two of its staff to jail for spying on Greenpeace campaigners, reports BBC News.
The company is hoping to build a new fleet of nuclear reactors in the UK.
A court in Nanterre, near Paris, found that EDF employed security firm Kargus to spy on Greenpeace as it campaigned against new reactors in France.
The court also sent two Kargus employees to jail and handed Greenpeace 500,000 euros (£428,000) in damages.
Greenpeace's campaign targeted in particular the new reactor being built at Flammanville on the Normandy coast, one of the European Pressurised Water Reactors (EPRs) that EDF hopes to bring to the UK.
Adelaide Colin, communications director for Greenpeace in France, said the decision "sends a strong signal to the nuclear industry: no-one is above the law".
The Tribunal Correctionel de Nanterre heard that Kargus Consultants, then run by a former member of the French foreign secret service, had compiled a dossier on Greenpeace via means that included hacking into a computer belonging to former campaigns head Yannick Jadot.
EDF maintained that it had just asked Kargus to monitor the activists, and that the consultants had exceeded their remit.
Read more of this report from BBC News.