“The police should have shot the #greenpeace terrorists! They should not be given preferential treatment,” tweeted Maxime Buizard, spokesperson for the youth wing of the UMP, reports French daily Liberation.

The youth party has fired Buizard and condemned his remarks. He has since apologised to Greenpeace and tweeted “sorry, my previous tweet was stupid, I deeply apologise.”

On Monday, activists belonging to environmentalist group Greenpeace sneaked into several nuclear plants, saying it had exposed the “vulnerability” of French nuclear sites.

“In about 15 minutes the activists reached the heart of the plant, where the nuclear core and nuclear fuel are,” Sophia Majnoni, a Greenpeace nuclear campaigner, told journalists.

French authorities admitted to security “lapses” after the incident and vowed a full investigation, while President Nicolas Sarkozy denounced the activists as “irresponsible”.

Nine activists entered a nuclear plant at Nogent-sur-Seine near Paris, while another two were found in a plant in the south of France.

Energy giant EDF, which runs the nuclear plants that France relies on for two thirds of its energy, sought to downplay the incidents, insisting it had been aware of the intrusion at Nogent-sur-Seine from the start.