The UK and US have held high level talks on the possibility of putting a "Son of Star Wars" missile defence system on British soil.
The US has had mixed results from its missile tests
Downing Street stressed talks were at an early stage. The US is thought to favour a site in eastern Europe.
Russia has said the system, which will destroy missiles launched at the US, will trigger an arms race.
Anti-war groups said UK involvement was "ill-conceived", and other critics said it could make Britain a target.
A Downing Street spokeswoman confirmed that Prime Minister Tony Blair thought the discussions were a "good idea".
"We believe that it is an important step towards providing missile defence coverage for Europe," she said.
But hours later, US deputy chief of mission in London, David Johnson, seemed to rebuff these claims.
He told the BBC: "As we go forward there may be opportunities for us to talk to other countries about their needs.
"But right now we are concentrating on the Czech Republic and on Poland as the primary sites where we would be looking for this."
Shortly afterwards, the American embassy released a statement on Mr Johnson's behalf, saying any suggestion of a rebuff was "nonsense".
"We have been and will be in discussions with the British government as we develop our missile defence system and be open to opportunities for joint work as we go forward."
BBC defence correspondent Paul Wood said talks were continuing between the National Security Council and Britain's top foreign affairs adviser Sir Nigel Sheinwald.
The system uses radar and satellites to detect enemy missile launches and to guide interceptors to their targets.
Shadow defence secretary Liam Fox said the Conservatives would not oppose locating part of the system in the UK, but wanted to examine it in detail.
"We have had no details at all from the government despite asking a lot of questions in Parliament.
"If the government really do want to maintain what they regard as a bipartisan approach to defence in this country, they better start getting honest with the opposition," said Mr Fox.
The Campaign for Nuclear Disarmament (CND) condemned UK involvement and warned the system could enable the US to attack other countries without fear of retaliation.
CND chairman Kate Hudson said: "It puts them on the front line in a future war. The US must call a halt to this dangerous and provocative system, and Britain must not participate in it."
Poland has recently confirmed the US wants to use its territory to build part of its missile defence base.
The US has also asked permission from the Czech Republic and received the backing of Czech Prime Minister Mirek Topolanek.
In 2002, the US withdrew from the 1972 Anti-Ballistic Missile Treaty it signed with the Soviet Union.
It says a missile defence system could significantly reduce threats from so-called "rogue states" such as Iran and North Korea.
But Paul Ingram, of the British American Security Information Council, also warned that the system would make Britain a potential target.
And he told the BBC the success of the system was "a long way from being proven".
"Even if it did work, it would be tackling the wrong problem at the wrong time," he told the BBC.
"The proliferation of ballistic missile technology is not as racing away as we are being led to believe.
"It has no relevance at all when it comes to issues like the war on terror."
Meanwhile, US defence officials say one part of the missile defence programme based at RAF Fylingdales in North Yorkshire - an early warning radar system upgrade - is almost complete.
The government faced opposition in 2003 when it agreed the work could go ahead.