The Obama administration on Wednesday publicly signaled its growing concern about a possible British exit from the EU, just days before David Cameron sets out plans for a referendum on the issue.
US diplomats have privately warned for months that Mr Cameron risked putting Britain on a path to exit with his plan to renegotiate Britain's EU membership terms and put the "new settlement" to a referendum.
But Washington has now taken the unusual step of publicly briefing British journalists that it firmly believes the "special relationship" is best served by the UK remaining at the heart of Europe.
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Philip Gordon, assistant secretary for European affairs, made it clear that there would be consequences for Britain if it either left the EU or played a lesser role in Brussels. "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 said. "That is in America's interests. We welcome an outward-looking EU with Britain in it."