Nato plans to build ballistic missile defence shield in face of scepticism

 to spend around £ 170 million on the system.

But that sum, a Nato background document says, will only meet the cost of command-and-control networks which will link future national interceptor missile and radar sites to a separate Europe-based US system designed to protect its troops.

The Pentagon's April, 2010 acquisitions report placed the cost of a similar US system at $58.01 billion (£36 billion) – after budget constraints forced the killing-off of futuristic components like Boeing 747-mounted lasers.

Individual European states will have to decide what national missile-defence assets they can afford," a Nato official said, "we're only providing architecture it can be plugged in to."

David Betz, a defence expert at King's College, London, said he "did not expect European states to be willing to spend the kind of money we're taking about involves for the next decade or more."

"Its impossible to speculate on what the final costs will be," Dr Betz said, "but I think the US figures offer a useful guide."

Ballistic missile defence systems consist of three distinct elements. Aegis, now deployed by the US and some allies like Japan and Spain, uses ship-based radar and an interceptor missile called the SM3, to protect forces in the field. Aegis Ashore is a land-based version of this system which uses a ground-based radar sites that are to be built in Turkey.

The second element of the ballistic missile shield is called Ground-based Midcourse Defence. Data gathered from satellites guides interceptors towards their targets, which release an infrared-guided kill vehicle that locates and destroys the warheads mounted on hostile missiles.

Finally, the US operates the Terminal High-Altitude Area Defence system, as well as short-range missiles like the Patriot PAC3, to hit incoming ballistic missiles at their terminal stage – that is, shortly before they hit their targets.

Few countries have pursued territorial missile defence, instead focussing on protecting their counterforce capabilities – that is, ensuring that not all their nuclear weapons are eliminated by an enemy first strike.

Russia operates the Galosh interceptor, which is armed with a 5 megaton nuclear warhead capable of destroying all incoming threats in a 20-kilometre radius. This eliminates for interceptor missiles to be accurate, but at the risk of causing considerable collateral damage.

More than a few experts believe the science behind territorial missile defence isn't good enough to ensure all incoming warheads will be neutralised – which means adversaries like Iran will have to be treated as nuclear-weapons threats even if ballistic missile defence is in place.

George Lewis, a physicist at Cornell University, and Theodore Postol, a former advisor to the US navy who now works at the Massachusetts Institute of Technology, recently revealed that just two interceptor-missile tests had been conducted in realistic conditions – and both failed.

The Pentagon, they wrote, changed the parameters for "all subsequent planned flight tests so that the missile defences would never again be tested against realistic conditions." Iran, the scientists noted, appeared to have been taking steps to make its ballistic missiles harder to detect. Yousaf Ahmed Vahidi, its defence minister, recently said that fins had been removed from Iran's Qiam 1 ballistic missile, which decreases "decreases the possibility of it being [hit]." The US government rejects the criticism. A senior US diplomat who spoke to The Telegraph on the issue described the critics as "professional Cassandras." In November 2008, Lieutenant-General Henry A. Obering III, the then-director of the agency praised the accuracy of the interceptor missile systems, saying "not only can we hit a bullet with a bullet, we can hit a spot on the bullet with a bullet."