A Number 10 spokesman said: "The US wants an outward-looking EU with Britain in it, and so do we."
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.
The US State Department's top official in Europe said America wants Britain to remain part of the European Union.
Philip Gordon said the saying the EU has "increasing weight in the world" and any referendum in the UK would risk turning the country "inward."
The comments are timely, as Political Editor Tom Bradby reports: