A "star wars" inspired defence shield that will alert the US to the launch of ballistic missiles is expected to be operational from a North Yorkshire airbase before the end of the year.

The Space Based Infrared System is seen as the successor to Ronald Reagan's unrealised "star wars" programme, which would have detected Soviet missiles and shot them down using lasers.

The new system, which has cost more than $10bn, has taken almost two decades to build and has been subject to numerous delays and cost overruns.

The imminent completion of the project at the RAF Menwith Hill base near Harrogate has revived fears that space is in danger of becoming militarised, with Britain in the vanguard.

"I find this disturbing to say the least," said a local Labour MP, Fabian Hamilton. "It's as if they are saying this is a place in the US and you are foreigners. But hang on one second, this is a bit of Yorkshire. We have no idea what they are doing. If they are developing star wars we have a right to know."

Menwith Hill houses a major US military site that is shrouded in secrecy and is already known to provide a home for Echelon, the US eavesdropping system that intercepts communications from around the world.

The site houses 33 satellite dishes encased in giant "golf balls" called radomes that receive data from SBIRS's four satellites 24,000 miles above earth.

The UK site is linked to Buckley, the airforce base in Colorado, home to almost 100,000 military personnel and the 460th space wing of the US Air Force Space Command. The 460th provides "missile warning, missile defence, technical intelligence, satellite command and control, and robust aerospace communications" according to its website.

But its work is top secret and critics say even the UK government has little idea what happens at Menwith Hill, which has been run by the US National Security Agency since 1966.

When asked about the site, former defence secretary Bob Ainsworth insisted its use was "governed by the terms of the Nato status of forces agreement of 1951 and other confidential arrangements between the UK and US".

Questions remain as to whether the US has sought permission from the UK to relay data from Menwith Hill to Buckley. Hamilton asked: "If the position were reversed and there was an RAF airbase in Massachusetts can you imagine the American military saying 'You do what you like'?"

Confirmation that the SBIRS was now ready to go live at Menwith Hill was revealed only in a terse parliamentary answer from defence minister Nick Harvey, who confirmed: "SBIRS facilities at RAF Menwith Hill are ready for operation." Harvey added: "The specific operational dates are a matter for the United States, although it is likely all SBIRS facilities at the base will be operational by the end of 2011."

A second, yet-to-be-completed project, the Space Tracking and Surveillance System, using satellites, will track missiles after SBIRS has detected them.

According to peace campaigners, the systems are a key component in what the US military calls "full spectrum dominance" – of land, sea, air, space and information.

Campaigners claim the presence of the US technology at Menwith Hill heightens the risk of a pre-emptive attack on the UK as a means of disabling America's missile-detection system.

They complain that planning approval for major projects at the site, whose main contractors are the US arms manufacturers Northrop Grumman and Lockheed Martin, was granted without consultation, and questioned whether it breaches the Outer Space Treaty which prohibits space becoming a new platform for the arms race.

Brigadier General Roger W Teague, the air force's infrared space systems director, has hinted that the SBIRS has more uses than missile detection.

"It is far more than just missile warning," Teague acknowledged this month, sparking speculation it could be used to gather "technical intelligence" for use by spy agencies.

But, according to its critics, many of the claims for the system are over-blown. "SBIRS is just one part of the highly complex US missile defence system and is a long way off from being operational," said Lindis Percy, joint co-ordinator of the Campaign for the Accountability of American Bases. "The US has spent billions and billions of dollars trying to develop this system ever since Ronald Reagan's fantasy of 'Star Wars' in the 1980s. It is still a fantasy, unlikely ever to work and should be scrapped."

According to the CAAB, the US has around 6,000 military bases on its own soil and more than 1,000 worldwide.