The Greenpeace activists entered Sizewell B in Suffolk at about 0700 BST on Monday.

Activists then climbed up ladders and onto the roof of a 20-metre-high pump house and unfurled banners which read "No More Nuclear".

A spokeswoman for Suffolk Police said officers were standing by in case they were needed.

Activists recorded their entry into Sizewell B

Greenpeace spokesman Peter Roche said the action was part of a campaign to oppose government proposals to build more nuclear power stations.

Mr Roche told BBC News Online the group opposes stations such as Sizewell.

He said: "There's no place to dispose of the nuclear waste we already have - it seems crazy to plan to produce even more."

Mr Roche said that some of the Greenpeace campaigners who occupied Sizewell B had been "thrown off the site" by police, but that none had been arrested.

The plant is patrolled by private security guards.

A DTI spokesman said: "The role of nuclear power in the UK's energy policy is a matter for the White Paper which we intend to publish in the New Year."