By David Wastell in Naples and Colin Brown

Jack Straw, the Foreign Secretary, fuelled fears of a transatlantic rift over the European Union's defence policy yesterday when he refused to reveal how Washington had reacted to news of a controversial deal struck by Britain, France and Germany.

It is believed that Colin Powell, the US secretary of state, telephoned two European foreign ministers on Friday seeking urgent clarification of the deal, which will allow the EU to conduct its own military operations independently of Nato.

The plan will also be discussed by Nato defence and foreign ministers at meetings in Brussels this week, where critics are expected to include Donald Rumsfeld, the hawkish US defence secretary.

Mr Straw spoke to Mr Powell on Thursday, the day after the deal was struck in Berlin, but said his response would have to remain one of the "secrets of the confessional".

He refused to be drawn on whether the Bush administration was willing to back the plan. "There is a process of discussion to take place with our American colleagues and with other partners in Nato," he said.

One Nato diplomat, however, had already warned that the proposal - broadly welcomed by other EU foreign ministers meeting in Naples - was a "Trojan horse" that could undermine the transatlantic alliance.

Senior Washington officials are concerned that the EU taking on an independent defence role will weaken Nato, the cornerstone of Western defence for more than 50 years.

Earlier this month, President George W. Bush said that he trusted Tony Blair to "make the right decision" on EU defence.

Yesterday, Mr Straw insisted that the plan would not weaken Nato. "The conclusion that we reached last night was one in which Nato is accepted as the alliance par excellence of all participating member states, the prime means by which we guarantee each other's mutual defence," he said.

Any EU defence activities would be "complementary" to the alliance.

Michael Ancram, the shadow foreign secretary, said: "This is a sell-out by Tony Blair. I think George Bush will be very angry and upset by this deal. They promised the Americans there would be no planning capability outside Nato. But that is what they have agreed."

Mr Ancram said he had warned President Bush about the initiative during his recent meeting with Michael Howard, the Conservative leader.

"President Bush told us that Tony Blair had assured him it would not undermine Nato. He said, 'I trust him and therefore I am not listening to you'. I think he will see this vindicates what we were saying," said Mr Ancram.

EU diplomats said that the defence plan, described by Dominique de Villepin, France's foreign minister, as "a breakthrough", would see a skeleton staff of about 30 "operational planners" established alongside an existing strategic planning unit in Brussels.

They would be put to work whenever the EU decided to deploy its own military force.

Nato officials are keenly awaiting further details of the plan, which was agreed during three-way talks in Berlin on Wednesday and put to EU ministers over dinner on Friday by Joschka Fischer, Germany's foreign minister.

The Nato officials will scrutinise the proposals for any signs of duplication of effort. One said: "We do not have a formal paper yet. We are anxious to see what they have agreed."

A Nato diplomat added: "You start with 30 people in a planning cell and end up with 300. It's a bridgehead to something much bigger."

The deal came as the EU foreign ministers made limited progress on other disputes that must be settled if an EU constitution is to be agreed in two weeks' time - a deadline set by Silvio Berlusconi, the Italian prime minister.

Diplomats put the chances at no greater than 50:50, and Mr Fischer said he was "deeply concerned" about the prospect of a successful conclusion.

A controversial plan - designed to make the commission less unwieldy - that would end the guarantee granted to each member state to appoint a member of the European Commission is close to being scrapped after protests from smaller countries.

There was, however, broad agreement on a separate proposal for a powerful, single EU "foreign minister", bolstered by a team of diplomats and based at a centralised foreign affairs ministry in Brussels.

After what British ministers see as a vital concession, the new supremo will be chosen from among the members of the European Commission, but will not take instructions from it.

Instead, he will come under the control of the Council of Ministers and thus, effectively, of the member governments.

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