LONDON — The United States said on Wednesday it wanted to see Britain stay in the European Union (EU) and have a strong voice in it, warning that a possible referendum on the country's membership of the 27-nation bloc could turn it "inwards."

The comments come as British Prime Minister David Cameron prepares to deliver a long-awaited speech on Britain's ties with the EU amid mounting calls for a referendum on whether to withdraw from the bloc.

"We have a growing relationship with the EU as an institution, which has an increasing voice in the world, and we want to se a strong British voice in that EU," said Philip Gordon, U.S. assistant secretary for European and Eurasian Affairs.

His comments were confirmed by two journalists present at a briefing on the subject.

Cameron says he wants to renegotiate ties with Europe and to then seek the public's "fresh consent" for a new settlement. In the coming weeks, he is widely expected to offer a referendum on Europe that could be held after the next general election in 2015.

Cameron says he does not want to leave Europe, a major trading partner, but euroscepticism has increased among Conservative lawmakers and the public in recent years, with the anti-Europe UK Independence Party making gains in local polls in 2012.