The disclosure risked infuriating Left-wing Labour MPs and prompting a fresh examination of transatlantic links and the relationship between Tony Blair and President George W Bush.

Lt Gen Trey Obering, head of the US Missile Defence Agency, said Britain was one of three candidates to be the European host of interceptors designed to shoot down incoming ballistic missiles.

British officials were startled by the disclosure, insisting that, as far as they were concerned, nothing had changed since Geoff Hoon, then the defence secretary, told parliament in 2004 that a decision to base interceptors in Britain would be "open to scrutiny and debate in the normal way".

"No one asked us the question [whether Britain was now ready to be a formal candidate]," a British Embassy spokesman said.

Previously, it was privately acknowledged that Britain is a strong candidate but it has never been formally identified as being in the running.

The revelation came in a PowerPoint presentation to a military conference earlier this week when Lt Gen Obering identified Britain, Poland and the Czech Republic as the three candidates, Britain seeming to have replaced Hungary on the final shortlist.

The Pentagon and the Ministry of Defence denied there had been any formal discussions over a missile defence site in the UK.

"There has been no planning, no approach no request and no invitation," a Pentagon spokesman said.

"America has not yet decided to place interceptors in Europe, has not yet asked for further British participation and the government has not yet decided whether or not to pursue missile defence for the UK," an MoD spokesman said.

Britain already plays a key part in American plans for a global defensive shield through the radar station at Fylingdales on the North York moors.

The planned missile defence shield, intended to protect America and its allies, has been dubbed "son of Star Wars", in a reference to the nickname for the Reagan-era defence system.

The Bush administration is now heading for a decision on where to put its third base of interceptors. The other two are in Alaska and California.

Sources close to Lt Gen Obering played down the impact of the disclosure saying Britain had two just brief mentions in his presentation.

He told the 4th Annual US Missile Defence Conference on Monday that the system which is to integrate land sea and air-based defences is making swift progress.

"It's a very daunting challenge but one that I think the men and women of the Missile Defence Agency, the army, navy and air force are pulling together and are now achieving," he said.

"A lot of people wonder if this is going to work and is it worth the investment," he added. "The testing we've conducted shows the technology is valid and viable."

He cited Iran and North Korea as potential threats but added that America and its allies must be ready to confront terrorist networks and future rogue states.

"Pakistan, one of our key allies today, tomorrow could have a fundamentalist Islamic government controlling their nuclear-tipped missiles," he said.

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The moves are likely to attract more unwelcome attention for prime minister Tony Blair's government over the nature of the transatlantic relationship.

Lt Gen Trey Obering, head of the Missile Defense Agency, disclosed at a military conference in Washington this week that the US had formally selected Britain as possible site for the interceptors in Europe.

However, Bryan Whitman, Pentagon spokesman, said on Wednesday that negotiations had not taken place on basing interceptors in the UK. "[There has been] no planning, no approach, no request, and no invitation."

Poland and the Czech Republic were already known as candidates, while Hungary has apparently been dropped. The Bush administration wanted to make a single choice soon, analysts said.

The ballistic missile defence system, which involves putting interceptors in Europe by 2010, is intended to protect the US from missile attack with land, sea, air and space-based assets.

The programme has cost $90bn since 1985 and the Pentagon plans to spend another $58bn in the next six years, according to a recent congressional report, which also highlighted test failures and criticised cost overruns and lack of transparency.

Britain already plays a crucial role in the defence of the US through the early-warning radar system at the Fylingdales base in northern England. The two sides signed a memorandum of understanding on ballistic missile defence in 2003.

Rumours of secret deals to place interceptors in the UK have been around for some time. In October 2004, Geoff Hoon, then UK defence secretary, told parliament any such decision would "be open to scrutiny and debate in the normal way" but that specific parliamentary approval would not be required.

Lt Gen Obering said the mission was to develop an "integrated layered" system that would also protect US allies.

The UK would be the "third site" for interceptors after Fort Greely in Alaska and Vandenberg Air Force Base in California.

Placing interceptors in the UK would raise the risk of Britain itself coming under attack, warns the British American Security Information Council (Basic), an independent defence watchdog.

Ian Davis, executive director of Basic, said it was inconceivable that the US would have announced the UK as a possible location without having discussed the issue with London. He thought eastern Europe was a more likely choice.

There was no immediate comment from UK officials.

Additional reporting by Demetri Sevastopulo in Washington