Joao Vale de Almeida was this week formally installed as the EU's ambassador to the US, and suggested that American officials should regard him as their first point of contact for transatlantic discussions.
He is the first EU ambassador to be appointed after the controversial Lisbon Treaty gave the EU sweeping new powers.
The prospect of an EU official speaking for the UK to its most important ally has angered eurosceptics, who said it shows Britain's waning influence.
Mr Vale de Almeida's comments are also likely to alarm Sir Nigel Sheinwald, the British ambassador in Washington. Sir Nigel is thought to have struggled to build strong relations with the White House after a memorandum he wrote for Gordon Brown questioning Barack Obama's experience was leaked in 2008.
Earlier this week, The Daily Telegraph disclosed that the Ministry of Defence had hired Washington-based lobbyists to supplement British Embassy staff.
Mr Vale de Almeida has stressed to Washington officials and politicians that under the EU's' Lisbon Treaty, he has more power than his predecessors. "I'm the first new type of ambassador for the European Union anywhere in the world," he said. "I'm supposed to have a wider mandate than my predecessors." Mr Vale de Almeida said: "Our delegations now cover a wide spectrum of issues well beyond the economic dimension, trade dimension and regulatory dimension, to cover all policies in the union, including foreign policy and security policy."
The Lisbon Treaty took force last year, taking the EU another step closer towards acting as a single entity in international affairs. The treaty created a European president and foreign minister and gave EU diplomats new powers to speak for all 27 union members on many issues.
In a comment that has come to symbolise the American view of the EU, Henry Kissinger, the former US secretary of state, is once said to asked: "When I want to talk to Europe, who do I call?"
In a response to that question, Mr Vale de Almeida declared: "In this area code, you call me." The ambassador insisted that he did not wish to "impose myself" on member states' ambassadors, who will continue to oversee "bilateral matters." But he declared: "Where we have a common position, I am the one leading the show."
The Lisbon treaty has led some politicians to suggest that the EU should take the place of countries like Britain and France at international bodies including the United Nations and the International Monetary Fund.
Mr Vale de Almeida, who is Portuguese, is one of the European Commission's longest serving officials. He was previously chief of staff to José Manuel Barroso, the Commission's president. Before leaving for Washington, the new ambassador was the most senior official under Baroness Ashton, the foreign-affairs commissioner.
The Earl of Dartmouth, a UK Independence Party MEP, said the ambassador's comments where proof that the EU diplomatic service is usurping the powers of nation states.
He said: "All the other parties have supported or accept the domination of the EU through the Lisbon Treaty. And this is the result. The British Government will be left discussing how British actors are portrayed in Hollywood at best." He added: "Of course Britain will continue to pay for the privilege.
This is only the start, as the existence of Britain's permanent seat on the UN Security Council is already in the EU sights".