France's state energy firm EDF has been fined €1.5m by a Paris court for spying on Greenpeace.

Its head of nuclear production security in 2006, Pascal Durieux, was given a three-year sentence with two years suspended, and a €10,000 fine for commissioning the spying. The Nanterre court also sentenced the security No 2 in 2006, Pierre-Paul François, to three years, 30 months suspended.

EDF has also been ordered to pay €500,000 in damages to Greenpeace.

The judge further handed down a guilty verdict on Thierry Lorho, head of Kargus, a firm employed by EDF to hack into Greenpeace's computers. He got three years in jail, two suspended, and a €4,000 fine.

EDF is the world's biggest nuclear energy supplier; it owns the UK nuclear power operator, British Energy, and is a major sponsor of the London Olympics. It was charged with complicity in concealing stolen documents and complicity to intrude on a computer network.

EDF and Greenpeace have fought for years over France's power production, more than three-quarters of it nuclear. According to confidential court testimony released by a French website, Mediapart, two years ago, EDF had organised surveillance not only of Greenpeace in France, but broadly across Europe since 2004.

In 2006, EDF hired a detective agency, Kargus Consultants, run by a former member of France's secret services, to find out about Greenpeace France's intentions and its plan to block new nuclear plants in the UK. The agency hacked the computer of Yannick Jadot, Greenpeace's then campaigns director, taking 1,400 documents.

An EDF official had no immediate comment. In the trial, EDF said it had victim of overzealous efforts, and had been unaware anyone would hack a computer. "The fine and the damages awarded send a strong signal to the nuclear industry that nobody is above the law," said Adélaïde Colin for Greenpeace France. "In the runup to the next presidential elections … voters should keep this scandal in mind."

The outrage among anti-nuclear campaigners echoes that which emerged when it was revealed that France's secret services were behind the bombing of the Greenpeace ship Rainbow Warrior 26 years ago. Moreover, safety is a very live issue since Japan's Fukushima disaster. In March, only weeks later, EDF boss Vincent de Rivaz said its plan to build four reactors in the UK would be unaffected, starting at Hinkley Point in Somerset.

At present, the four EPR are being built in Finland, France and China are well behind schedule, hampered by construction problems and billions over budget, in the case of EDF's reactors in Finland, and France.

Speaking from alongside the new Rainbow Warrior, currently on its maiden voyage and docked in London, Greenpeace UK's executive director, John Sauven, said: "The evidence presented at the trial showed that the espionage undertaken by EDF in its efforts to discredit Greenpeace was both extensive and totally illegal. The company should now give a full account of the spying operation it mounted against its critics. As one of the six companies with a monopoly over electricity supply in this country and a major sponsor of the Olympics, EDF has a duty to come clean. The length of the sentences handed down shows just how seriously the judge views what the French state owned company did."

Yesterday, EDF raised its target for nuclear generation in France and reported a 3.2% rise in sales.