"Do we feel the need for a separate EU military headquarters? No," an official spokesman for Prime Minister Tony Blair told reporters at an EU summit in Brussels.
"The defence of Europe is a matter for NATO, not for the European Union," he added, stressing that the 19-nation military alliance remained Europe's bulwark against external threats.
The spokesman said the EU should continue work on strengthening its own military initiatives without undermining the primacy of NATO.
"How that is put together is a matter for further cooperation."
In April, Belgium, France, Germany and Luxembourg proposed setting up an EU military HQ at Tervuren, on the outskirts of Brussels, to enable the Union to plan its own operations "without recourse to the means and capacities of NATO".
Britain has denied it agreed to back the HQ plan when Blair held talks with the French and German leaders in Berlin last month.
Blair and French President Jacques Chirac were expected to hold informal talks on the sidelines of the two-day EU summit, French sources said.
According to the Financial Times newspaper on Thursday, Washington has reacted with dismay to "mixed signals" from Downing Street about the EU defence plans.
It said US national security adviser Condoleezza Rice had telephoned Nigel Scheinwald, Blair's chief foreign policy aide and formerly Britain's ambassador to the EU, to express her concern.
Blair's spokesman said Foreign Secretary Jack Straw "does not recognise the picture painted in the FT".
"We do not recognise the level of concern expressed in the FT story," he added.