A spokesman for the Russian Foreign Minister, Sergey Lavrov, warned that the move would "represent a disturbing new step" in international relations and posed a "possible threat to the security of Russia".

The Foreign Ministry also hinted heavily that siting US missiles in Britain or elsewhere in Europe could lead to a new arms race with the US - a threat that will alarm ministers and opponents of the proposal.

Moscow's criticisms came after the IoS disclosed that Tony Blair has privately given President George Bush an agreement in principle to host interceptors in Britain as part of the US "ballistic missile defence" system.

The true extent of Britain's deepening co-operation with the Pentagon on bringing the system to the UK was revealed in an official document on missile defence released by the Secretary of State for Defence, Geoff Hoon, last week.

It confirms that detailed studies into how the "son of Star Wars" weapons system would extend to cover Britain have begun. They include how and where US missiles will be sited and how effective they would be in shooting down an enemy's ballistic missiles.

These further disclosures led Nicholas Soames, the Shadow Defence  Secretary, to demand in the Commons last week that Mr Hoon "come clean" about the Government's thinking. As ministers continued to insist no formal decision had been taken, Mr Soames claimed the Government had a history of taking sensitive military decisions in secret without informing MPs.

The "son of Star Wars" agreement - signed in June last year by the MoD's chief scientist, Professor Roy Anderson, and the head of the US Missile Defence Agency, Lieutenant-General Henry Obering - confirms the MoD is working on the "development and analysis of options for the extension of the US system to make missile defence capabilities available to the UK".

It also reveals that the MoD is investigating whether shooting down nuclear-armed, biological or chemical weapons over the UK could contaminate Britain or other parts of northern Europe with fallout.

''We possess no official information on that score. But if such a joint decision of the United States and Britain did take place, it would represent an alarming new step towards an escalated deployment of the US missile defence system,'' a Russian Foreign Ministry statement said, commenting on the report in a section of the British press, that Prime Minister Tony Blair has given a secret consent to US President George W. Bush to deploy US missile interceptors at the RAF Fylingdales radar base in North Yorkshire.

''This kind of step would have to be viewed in relation to other elements of a US plan to deploy missile defence components in Europe-to station a group of ground-based long-range interceptor missiles and a missile defence radar station within certain East European countries and to upgrade the radar stations in Greenland and in Britain for missile defence purposes,'' the statement pointed out.

''So this involves setting up already two new missile defence bases outside the US national territory, which by virtue of their geographical position, can pose a threat to Russia's nuclear deterrence capability,'' the statement stressed. The statement said that although the US gave Russia assurances that the missile defence system now being established-together with its foreign bases-was not aimed at Russia, the questioned remained about how that ''not being aimed" will be provided and assured.

''So long as there is no answer to the question, the Russian side cannot but take into account a possible threat to the security of Russia,'' it noted. ''We are going to carefully analyse the situation and, if necessary, will take adequate measures to ensure our security,'' the statement warned.