Mr Hoon continued: "I am therefore today replying to the United States Secretary of Defence Donald Rumsfeld, conveying the government's agreement to the US request."

The news is set to anger opponents of the plan, who have already complained there has only been scant consultation about the US request.

The Commons defence select committee has also pointed to a lack of proper consultation with residents near the North York Moors base.

Upgrade details

In his statement, Mr Hoon said the upgrade would not change the external appearance or power output" of the base.

There would also be no new development, but computer software and hardware would change.

Mr Hoon added: "The radar will continue to fulfil its long-established ballistic missile early warning system role, it will continue to be operated and staffed by the Royal Air Force and we will continue to enjoy full access to its data.

"The upgrade does not of itself commit the UK Government to any greater participation in the US missile defence programme.

"It does, however, keep open the prospect of acquiring missile defence capabilities for the UK, should we desire such protection at some point in the future.

"We will continue discussions with the local planning authorities on the detail of the upgrade work."

The Conservatives favour the scheme, but said it was typical the government had "smuggled out an unpopular announcement".

Shadow defence secretary Bernard Jenkin said: "They stifled debate on missile defence until the Christmas holidays, and then rushed out the policy through with the minimum of debate.

"The whole process stinks of hypocrisy."

The Liberal Democrats have already accused ministers of undermining their own consultation process by moving too quickly.

Last month, Mr Hoon told MPs a threat of a ballistic missile attack by rogue states could materialise.

He said the missile system could represent "an invaluable extra insurance against the development of a still uncertain but potentially catastrophic threat to the citizens of this country".

Independence threat?

Opponents say the scheme is unnecessary, would not work and would simply make the UK a target - without giving it a protective screen of missiles.

Former Labour Defence Minister Peter Kilfoyle has attacked what he termed Mr Hoon's "slavish devotion to American policy in this area".

In the last debate, Mr Kilfoyle spoke of "the government's gathering rush to embrace every crackpot notion foisted upon us by the ideologues in Washington".

Fellow Labour MP Alice Mahon told Mr Hoon that government policy was out of kilter with the opinion of the vast majority of people who elected Labour.

'Expensive and dubious'

Fylingdales has an advanced radar which could track missiles launched by rogue states or terrorists in, for example, the Middle East.

They would then in theory be shot down as they neared the US coast.

Labour MP Malcolm Savidge has described the system as an "extremely expensive and technically dubious defence against a very improbable form of attack".

In December, the government published a report arguing that taking part in the US plans would not immediately put the UK itself at greater risk of attack.

And in November the US head of the missile defence Lieutenant General Ronald Kadish, visited the site on a "familiarisation" trip.