"While Tony Blair rattles his sabre and waves international treaties at foreigners, he's agitating for Britain to break those same treaties," said Louise Edge, a campaigner for the environmental group Greenpeace.

"That's why hundreds of people from across Britain have come here to inspect our country's bomb-making factory and highlight Tony Blair's hypocrisy," she said.

Lawmakers in Britain are to vote on replacing its Trident nuclear deterrent missiles early next year, although Blair has indicated that he is in favour of an update.

"We are going to take photographs and pin legal advice to the fence suggesting that the government's Trident renewal would be illegal according to international law," a Greenpeace spokesman said.

He said the advice comes from Philippe Sands, a lawyer championed by Greenpeace for his criticisms of Blair's desire to replace Britain's US-built, submarine-based nuclear missiles which are due to become obsolete in two decades.

Greenpeace said Sands, who works at Matrix Chambers, where Blair's wife Cherie is a leading human rights lawyer, warns that the prime minister's approach could violate the Nuclear Non-Proliferation Treaty.

Renewal, replacement or upgrading of Trident is likely to breach Article VI of the Nuclear Non-Proliferation Treaty (NPT), according to Sands.

Thames Valley Police said their officers were guarding the base near Reading, west of London, to ensure the protest stayed peaceful.

In a speech later in London, former UN weapons inspector Hans Blix is to argue that updating Trident would put the NPT under "strain" and would make it more difficult to stop Iran acquiring nuclear technology, The Independent reported.

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