The Obama administration has expressed concern about the impact of a UK referendum on its future relationship with the EU, making clear that it is in America's interests to see a "strong British voice within the EU".
"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”, said Philip Gordon for the US State Department. He added "we welcome an outward-looking EU with Britain in it."
For the Prime Minister, No 10 responded: "The US wants an outward-looking EU with Britain in it, and so do we."
Mr Cameron wants the UK to remain within the EU but believes there is a need to redefine the relationship in light of moves towards further integration by countries using the single currency. Mr Cameron has suggested that the British electorate would be asked to give "fresh consent" for any new deal that emerges as a result of negotiations with other EU countries.
However, many Conservative MPs want him to commit to a referendum on the question of whether the UK remains in the EU or not - a so-called "in-out vote" - which he does not support.
Mr Gordon made clear he was talking about US interests, and that "what is in the UK's interests is up to the UK".
Earlier in January, ten of the UK's leading business figures warned Mr Cameron not to put membership of the EU "at risk" in negotiations over the country's relationship with Brussels.
Shadow foreign secretary Douglas Alexander said: "On the same day that top British business leaders fired a warning shot across the bows of the prime minister, a senior White House official has also signalled concerns about the possibility of Britain leaving the EU.
"There is today a real risk of Britain sleepwalking towards exit because of a prime minister motivated more by the need for party unity than by the interests of the country."
BBC News (9th January): www.bbc.co.uk