US warns Britain on EU referendum

He added that "referendums have often turned countries inwards" and that it was in America's interest for Britain to stay in an "outward-looking EU."

The warning from Britain's big overseas ally and partner comes ahead of a eagerly awaited speech by David Cameron on redefining his country's relationship with the EU.

Cameron on Wednesday confirmed to the House of Commons that he still wants Britain to be "involved in the single market" and be an "active" player in the EU.

But he added that because of changes to governance of the eurozone it was necessary to have a fresh settlement with the EU and then hold a referendum on it.

Previously he had said he would seek to repatriate powers from Brussels as part of this new arrangement, but a large part of his Conservative party wants to go further and is seeking a public poll on the matter.

But other leaders have indicated strong reluctance towards treaty change which would also imply referendums in some member states, such as Ireland.

"Maybe treaty change will come for a variety further down the road, but now I don't see an issue of opening the treaty for change for any individual country, because it would open the floodgates for requirements for changes from many other countries," said Irish leader Enda Kenny, currently chairing the rotating EU presidency.

Kenny also noted that the Irish presidency sees it as a matter of priority to negotiate an EU-US free trade agreement, which "from the British perspective would be very important to Prime Minister Cameron."

For his part, EU council president Herman Van Rompuy said he is waiting for Cameron to deliver his speech and that it was "in Britain's interest to stay not only as a member, but a very active and full member, a leading nation in the EU."

As for treaty change, he said the only possibility for it would be after the 2014 European Parliament elections, but for now there was no consensus on any of the issues requiring such a move.