The base is the largest electronic monitoring station in the world

Defence Secretary Des Browne said the system was "a building block to enhance our national and collective security".

CND said it was "outraged" by the move which it said had been done without consulting MPs or the public.

Mr Browne also said a radar upgrade at RAF Fylingdales near Whitby, started in 2003 by the US, was complete.

The Fylingdales radar is expected to switch operations to the new equipment next month.

"There is no change to the existing UK-US mission for the radar and the station remains under full UK command," added Mr Browne.

"The UK will have full insight into the operation of the US missile defence system when missile engagements take place that are wholly or partly influenced by data from the radar at RAF Fylingdales."

Warning data

Mr Browne said the work at RAF Menwith Hill would support the existing UK-US missile warning mission and enable satellite data to be passed into the new US missile defence system.

This totally unnecessary system will protect no-one, but threaten all CND chairwoman Kate Hudson

"Equipment will be installed and operated by the US government to allow receipt of satellite warnings of potentially hostile missile launches, and will pass this warning data both to UK and US authorities.

"The data will also be fed into the US ballistic missile defence system for use in their response to any missile attack on the US," Mr Browne said.

"This will guarantee the UK's continued access to essential missile attack warning data, as well as enhancing the US's ability to deal with any attack aimed at their country."

'Outrageous'

The receiving antenna equipment already exists at RAF Menwith Hill and installing communications hardware is the only extra work required.

But anti-nuclear campaigners CND accused the government of "whipping up tensions in Europe in order supposedly to fight a threat that does not exist".

CND chairwoman Kate Hudson said: "This is an outrageous announcement made with total contempt for democracy and consultation.

"Where was the chance for MPs, let alone the public to discuss tying in Menwith Hill to the so-called missile defence system?"

"It looks like the MoD and the US have deceived Parliament and deceived the British people when previously the minister dismissed suggestions that developments at Menwith Hill would form part of NMD."

She said Mr Browne was "disingenuous" to suggest missile defence systems were not for offensive use.

"This totally unnecessary system will protect no-one, but threaten all - by giving the US the ability to strike with impunity without any fear of retaliation," she added.

CND also disputed the government's claim that Menwith Hill would remain under UK command, saying many of the personnel based there were from America.