Tony Blair has suffered a major rebuff after the Americans poured cold water over an offer to site key elements of their controversial "Son of Star Wars" missile defence system on British soil.

Earlier, No 10 confirmed for the first time that the Government was in discussion with Washington about hosting parts of the programme - believed to be the interceptors used to bring down a ballistic missile - in the UK.

Within hours, however, the US deputy chief of mission in London, David Johnson, went on the airwaves to say the US administration was primarily focusing on allies in Eastern Europe to locate the system.

"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," he told BBC Radio 4's The World at One.

The rejection of the UK to host the programme is likely to be seen as a massive embarrassment for Mr Blair, after No 10 had made clear that Britain had come forward with the offer.

"It is our intention that whilst the United States are in the decision-making process, the UK should be considered as part of that," a spokeswoman said.

"The Prime Minister thinks it is a good idea that we are part of the consideration by the United States. We believe that it is an important step towards providing missile defence coverage for Europe of which we are part."

No 10 would not expand on why Mr Blair wanted it to be located in Britain. However, the offer appears to have been an attempt to further bind the UK to the US before he leaves office.

The Americans are primarily searching for a European site for the underground silos which would house the interceptor missiles used to "shoot down" a ballistic missile as it hurtles through space on the way to its target.

The aim is to provide protection from a missile launched from a "rogue state" in the Middle East against the US, its allies, or its forces overseas.