But work on upgrading the key Fylingdales radar installation in Yorkshire as part of the US early-warning system, for detecting attacks by rogue and potentially nuclear states such as Iran or North Korea, will continue at American expense.

The White House is now looking at either Poland or the Czech Republic as possible locations for the installation of a battery of 10 anti-missile weapons by 2011.

The UK would remain in the loop as one of four crucial global detection centres providing data to guide the 155lb "kill vehicles" to a point in space where they could destroy incoming ballistic missiles.

A Pentagon official admitted yesterday the declining domestic political fortunes of Tony Blair and growing opposition to his support for the war in Iraq had been factors in deciding not to ask the UK to host the interceptors.

"There is no serious discussion at this time about installing anti-missile interceptors in Britain," he added.

The limited anti-missile system, nicknamed Son of Star Wars because of its origins in the far more ambitious military space race of the 1980s, would be incapable of knocking out massed salvos of nuclear missiles from Russia or China.

Planners have scaled back the original aim of the Ronald Reagan era to design a system capable of intercepting the handful of missiles which might be launched by states like Iran or North Korea.

Fylingdales is already part of the cold war chain of nuclear attack early-warning stations, which includes Thule in Greenland and Clear, Alaska.

The US National Security Agency also runs RAF Menwith Hill, near Harrogate, to co-ordinate data on missile launches detected through infra-red scanning by US satellites orbiting high in space.

Fylingdales's current role is to track incoming missiles, evaluate their trajectories and calculate where they will land. Its new function will be to translate that tracking into targeting information fed to the interceptors.

The Ministry of Defence confirmed there had been no discussion on the siting of missiles on UK soil.

A Commons defence committee study three years ago predicted that buying into the missile defence umbrella would cost up to £10bn.

Military sources said the US interceptors, already based in Alaska, had performed poorly in trials.

It would be "at least a decade" before either Iran or North Korea developed intercontinental missiles capable of reaching either the west coast of America or central Europe.