Tony Blair said after a total of 30 environmentalists got into the plant that nuclear power stations’ security should be reviewed.

Rob Gueterbrook, one of the campaigners, said: “It’s easier to get into than a Norwich nightclub. It is a terrifying thought that if we can do this then anyone can. If terrorists targeted a nuclear power station it would be deadly.”

Greenpeace broke into the site shortly after 6am by cutting their way through a fence and using ladders to gain access to different areas. Some entered a central control building, while others clambered on the roof, painting the word “Danger” on the side of the dome. They said that no alarms were raised when they broke in.

Their spokesman, Blake Lee-Harwood, said: “We managed to walk in to this flagship site with almost no equipment and can get to the top of the dome, which is the icon of this installation. It shows how easy it is to gain access.” Others said that they had access to controls when no staff were around. The campaigners were eventually escorted from the premises by police and security guards.

Mr Blair said: “I am sure those in charge of security will review any lessons to be learnt. There is a continuing threat and that’s the very reason why this is so important to deal with.”

It is the second time in four months that Greenpeace protesters have broken into Sizewell B. In October 140 volunteers broke in, and two women and five men camped overnight on the roof of a cooling tower before coming down voluntarily.

Mike Harrison, maintenance manager at Sizewell B, insisted that security had been tightened after the last break-in. “At no time was there any risk to plant safety or public safety,” he said. “We have taken steps and have beefed up security in several areas.” A spokesman for British Energy condemned the campaigners and also insisted that there was no danger: “There are well-rehearsed security arrangements in place.”

Oliver Letwin, the Shadow Home Secretary, said the break-in raised serious security concerns. “I am horrified that protesters were able to break into Sizewell B. What would we be saying if these had been terrorists? What is going to be done to make this absolutely impossible in future?”

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