Britain risks damaging its relationship with America and being sidelined in the international community if it leaves the European Union, the Obama administration has warned.
"We value a strong UK voice in a strong European Union," the US State Department's Philip H Gordon, the Assistant Secretary for European and Eurasian Affairs, said starkly during a visit to London to meet ministers yesterday.
"We have a growing relationship with the EU as an institution, which has an increasing voice in the world, and we want to see a strong British voice in that EU.
"That is in America's interests. We welcome an outward-looking EU with Britain in it."
A British exit from the EU would not enhance the "special relationship" in any way, he said. America would continue to forge stronger links with member countries of the EU, which it sees as having "a growing voice in the world and a critical partner on global issues".
The public statements of Mr Gordon, a respected senior member of the administration, show the level of concern in Washington over a mooted referendum on British membership of the EU.
David Cameron has told the Commons that although it was not his "preference" to pull out of the EU, he could see that doing so would be "imaginable".
In a long-awaited speech due soon on Europe, the Prime Minister, it is believed, will say that a referendum will take place in 2018, after the next election.
But Eurosceptics have demanded that it is held sooner and Boris Johnson, seen as a rival for the Conservative leadership, has joined them in making the call.
The Mayor of London said recently that it would be "fantastic" if the vote were taken sooner, and declared that "I don't think that leaving the EU is the end of the world."
François Hollande, the French President, has, in turn, stressed that the UK cannot pick which laws it likes from an "à la carte" EU and should realise that membership "is for life".
Mr Gordon said: "We have a growing relationship with the European Union as an institution which has a growing voice in the world and we want to see a strong British voice in that European Union."