US President Barack Obama has warned the UK that it would lose influence if it left the EU.
A referendum on whether to remain in the EU would be held by the end of 2017 under Prime Minister David Cameron's plans to renegotiate the terms of the UK's membership before the new deal was put to the public.
Obama said the UK's membership of the EU gave "much greater confidence" about the relationship between the US and Europe.
He said in a BBC interview that he wanted to make sure the UK continued to have the "influence" that came with membership of the EU.
He said, "Having the United Kingdom in the European Union gives us much greater confidence about the state of the transatlantic union and is part of the cornerstone of the institutions built after World War ll that has made the world safer and more prosperous.
"We want to make sure that the United Kingdom continues to have that influence."
It was the latest sign of the importance Washington placed in the UK continuing with the EU, after comments made by Obama at the G7 summit of world leaders in Germany in June.
As he met Cameron for talks at that summit, the president said that "one of the great values of having the United Kingdom in the European Union is its leadership and strength on a whole host of global challenges, so we very much are looking forward to the United Kingdom staying part of the European Union because we think its influence is positive not just for Europe, but also for the world".
According to commentators, Obama's comments would be welcome in Downing Street, where David Cameron would lead the referendum campaign for the UK to remain in the EU following his renegotiation of the terms of membership.
Although Cameron had not ruled out recommending withdrawal in the event of failure of his renegotiation, he was expected to commit to staying with the EU eventually.
According to pro-Europeans, leaving the EU would diminish the international influence of the UK, and they would be better able to make that case with Obama's comments.
However, the Eurosceptic Conservative MEP Daniel Hannan posted a message on Twitter saying: ''I accept that there may be some arguments for Britain staying in the EU. Humouring Barack Obama is not one of them.''
Also the Ukip MEP Patrick O'Flynn said Obama was unwise to pressure the UK to stay in the EU for the US's convenience. ''We need to look to our own national interest first,'' he said.