In an interview with The Telegraph, Mr Rumsfeld warns that the plans could "inject instability" into the Nato alliance and "put at risk something that is very special". It is the first detailed public statement on the subject by a senior American politician since Mr Blair returned from Camp David last month claiming that President Bush had agreed to support the European "army". 

Mr Rumsfeld makes clear that the new Republican administration remains deeply concerned about the project. Invited to confirm that he is "relaxed" about the European Union's proposals, Mr Rumsfeld conspicuously declines to do so. 

Mr Rumsfeld says: "I personally will be watching carefully to see how things evolve, because we have so much at stake with that [Nato] alliance. We need to be vigilant to see that we don't do anything that could inject an instability into the alliance."

 In a sign of his frustration that the controversy refuses to die down, Mr Blair, also interviewed The Sunday Telegraph, accuses the Conservative Party of pouring "poison" into the ears of the Americans. However, the Prime Minister makes the startling admission that some EU countries involved in the defence initiative may intend "to destroy Nato" - a reference that will be assumed to apply to France.

 Mr Blair says: "Well, if we don't get involved in European defence, it will happen without Britain. Then those people who really may have an agenda to destroy Nato will have control of it." The re-emergence of the transatlantic rift is a blow to Mr Blair who sought to alleviate Mr Bush's concerns when they met at Camp David a month ago. 

At the time, Mr Bush said: "He assured me that Nato is going to be the primary way to keep the peace in Europe." The President's advisers have, however, been alarmed by annexes to the Nice Treaty, signed last year, which state that the European force will be "under the political control and strategic direction of the EU" during operations.

 Mr Rumsfeld, interviewed in Washington by the former Conservative MP, Winston Churchill, was asked to confirm Mr Blair's view that the Bush administration was now "relaxed" about the force. Instead of agreeing, he replied: "I think the correct way to say it is that the President has said what he has said about it, and he understands it."

 Mr Rumsfeld says: "As in so many things in life, the devil is in the detail. And the details haven't been worked out. The way the planning mechanism is handled could make an enormous difference. But arranged in a way that didn't really look out over the long term . . . then it could put at risk something that's very special."

 In the interview, Mr Blair says: "They [the Bush administration] have had poison poured in their ear by the present Conservative Party going over there and saying, this is all about ripping apart Nato, it's a French plot to destabilise..."

 Mr Blair says: "Every time I explain European defence to Americans they understand it and end up supporting it. But this is all part of that ghastly [Conservative] traffic that goes across there saying, 'Oh, you know, the purpose of the New Labour Government is to pull Britain apart from America'."