Protestors are to besiege the Fylingdales missile early warning base in North Yorkshire on Saturday.

Demonstrators will swell support for existing peace camps after the Government confirmed yesterday that the US has officially asked to use the base as a crucial part of its "Son of Star Wars" National Missile Defence programme.

It coincides with an announcement from the Bush administration that it hopes to have the programme up and running by 2004.

Defence Secretary Geoff Hoon said Britain would "consider the US request very seriously", and agree to it "only if we are satisfied that it will ultimately enhance the security of the UK and the Nato alliance".

But peace protestors - who warn that helping the US programme will make Fylingdales a target for rogue states and terrorists - claim the decision to agree to use the base, on the North York Moors, has already been made.

Last week, Mr Hoon suggested Fylingdales might have to be upgraded anyway to improve protection.

Yesterday, he said there was no "immediate threat" to the UK from ballistic missiles.

But Mr Hoon added: "Intentions can change quickly, and the proliferation and development of weapons of mass destruction and ballistic missiles is continuing."

York Labour MP Hugh Bayley is urging Mr Hoon to to listen to local reactions before the Government responds to the US.

"What's in it for us?," he said. "If this is a system just to defend America, it will be a lot less attractive than one that also defends us and the rest of Europe."

Neil Kingsnorth, of Yorkshire CND, said: "The whole discussion is a public relations exercise.

"The decision has already been made."