Tony Blair has vowed not to back any European defence policies that could undermine Britain's relationship with America.

Speaking at his monthly press conference on Thursday, the prime minister said he was "absolutely the strongest ally that the United States could have".

Media reports over recent weeks have suggest that Bush administration is concerned that plans for closer European Union defence cooperation could undermine the key role of NATO.

But Blair said that British support for European defence cooperation was "not an issue between me and the [US] administration".

The prime minister said there should be no duplication of NATO's planning capabilities "and we certainly don't want competition".

He said the recent record on defence issues showed the EU was not preparing to challenge the US. "We have got no intention of doing that, we will never do that."

But he said he would not give up on efforts to ensure Europe can act "when America does not wish to be engaged".

"This is an article of faith with me and people have got to make up their minds whether they agree with it or they don't agree with it," he warned.

"But there are people who want to pull me apart from America, there are people who want to pull me apart from the centre of Europe.

"I will not yield up either pillar of Britain's foreign policy in the early 21st Century because it makes sense for Britain."

He said that the UK should be "strong in Europe, strong with America" in order to play an effective role with both continents.

The prime minister added that this position was "the right thing, I think, for my country".

"The slight problem I would say that any critic of mine has got on this front is that it is quite difficult to argue after the past year that I am a slothful ally of the US," Blair said.

"I am absolutely the strongest ally that the United States could have.

"But I know there will be certain situations, for perfectly good reasons, where the US doesn't want to undertake military operations and the EU in those circumstances has got to have the capability to do so."