The row over Britain's relationship with Brussels has intensified after the US expressed concern about the prospect of the UK moving to the sidelines of the European Union.
The unusually direct intervention by the Obama administration provoked a furious backlash from Conservative eurosceptics who want to see the UK loosen its ties with the EU. But Deputy Prime Minister Nick Clegg seized on the warning as evidence that it was in Britain's interests to "stand tall" in Europe.
As Westminster eagerly awaits a landmark speech on the EU by David Cameron, the US assistant secretary for European affairs Philip Gordon made clear Washington favoured a "strong British voice" in Brussels. He also warned that referendums could turn countries "inward".
"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," he told reporters during a visit to London. That is in America's interests. We welcome an outward-looking EU with Britain in it."
The Prime Minister is due to make a speech this month in which he will set out his plans to renegotiate Britain's relationship with the EU - including clawing back many powers - and put that settlement to voters.
Tory MP Bernard Jenkin said on Thursday that the US had not "got a clue". "The Americans don't understand Europe. They have a default position that sometimes the United States of Europe is going to be the same as the United States of America. They haven't got a clue," he told BBC Radio Five Live.
Another eurosceptic Tory MP, Peter Bone, said Mr Gordon should "butt out" and that it was "nothing to do with the Americans". He added: "It's like us trying to tell Germany or France how to run their affairs. It's quite ridiculous and it's not what you'd expect from a member of the senior executive in the USA, and I hope that the president will slap him down very quickly."
But Mr Clegg, in a phone-in on LBC radio, said that Mr Gordon's comments were "entirely unsurprising". "Americans have been saying for generations, for ages, since the 1950s, that Britain and the special relationship between Britain and America - which is a really important one, it's one we've relied upon through thick and thin, through conflict and peace - is one that is partly based on the fact that we are valuable to our American friends, and frankly we are also important to people in Beijing and Tokyo, because we stand tall in our own neighbourhood.
"If you want to lead around the world, and this is a globalised environment we are walking in, the first thing you've got to do is be strong in your neck of the woods. I think that's the point they are making." He added: "They are perfectly entitled to say, 'look, if you are interested in the American perspective, we think that Britain stands taller in the world if you stand tall in your own neck of the woods'."
A Number 10 spokesman said: "The US wants an outward-looking EU with Britain in it, and so do we."