"Forty people have come down off the roof and those people have now been arrested," a spokesman for the Suffolk police said yesterday evening, adding that four people had been arrested earlier in the day.

"We are continuing to liaise with those who remain on the roof in an effort to bring this situation to a peaceful conclusion," the spokesman said.

It was not clear how many remained on the roof yesterday night.

Greenpeace has stepped up its campaign against nuclear power since September, when British Energy was provided with emergency funding from the state to stave off insolvency.

A British Energy spokeswoman called the occupation at the site of the last nuclear power station to be built in Britain "a breach of security" and said the protesters had used ladders to scale a perimeter fence at the plant on the Suffolk coast near Ipswich.

"Safety is the paramount issue here and we are cooperating with the police to that end," she said.

Nuclear power provides about a quarter of Britain's electricity, but its stations are ageing and suffer from high costs and low flexibility in a newly liberalised wholesale power market that has sent prices down 40 percent since 1998.

Greenpeace argues that the possibilities for radioactive contamination and the costs of storing waste make the industry dangerous and uneconomic. It says the stations should be closed down as soon as possible and be replaced by wind energy and other renewable sources.

Others in the power industry say the shortfall from nuclear closures could not be made up fast enough and that the nation needs nuclear power for its electricity and to keep down the polluting carbon dioxide emissions produced by other traditional generators.