"Missile defence systems are just that -- defensive," Defence Secretary Des Browne said in a statement.

"They are not for offensive use and by supporting American efforts in this area, both through scientific cooperation and by allowing the use of facilities in the UK, we are helping to build future protection for our citizens," he said.

It was not clear precisely what the site at Menwith Hill, in North Yorkshire, would be used for, but the ministry said it would allow for threat detection and "enable satellite data to be passed into the new US missile defence system".

The decision follows an earlier agreement to allow the Americans to upgrade the radar system at another British base, RAF Fylingdales, as part of the missile defence system.

Russia has reacted angrily to US plans to site parts of the missile defence system in Poland and the Czech Republic.

Washington says the shield is designed to protect against attack from what it calls "rogue states" but Moscow sees the missile system as a threat.

Anti-nuclear campaigners have criticised the British government for lending support to the US missile defence plan, saying the system would enable Washington to attack other countries without fear of retaliation.

Rather than protecting British citizens, they argue that it puts British people at greater risk.

Browne said Britain would work with the United States and other NATO allies to look into the possibility of a NATO missile defence system that could build on the US programme.