By Peter Griffiths
LONDON (Reuters) - Britain would damage its standing in the world and miss out on lucrative trade deals if it turns its back on the European Union and votes to leave the 28-nation bloc, one of Europe's most senior figures said on Wednesday.
In a speech in London, European Council President Herman Van Rompuy said large countries such as Britain, the world's sixth biggest economy, had the most to lose if they decided to go it alone in the world.
"The United Kingdom's commercial outreach, its military and diplomatic clout are matched only by a few other countries," he said in a speech on Europe's global role at Regent's University.
"By working jointly, it has the most to win. For a country like the UK to make its voice heard in the world, Europe does not work as a damper, but as a megaphone."
The prospect of Britain leaving the European alliance moved a step closer in January when Prime Minister David Cameron promised to hold a referendum on its EU membership before the end of 2017, provided he wins the next election in May 2015.
His pledge to first negotiate a new deal for Britain in Europe and to claw back powers from the bloc infuriated many EU allies and drew warnings from Japan and the United States.
Cameron's policy was seen as an attempt to placate anti-EU forces in his Conservative Party and counter the threat posed by the rising UK Independence Party, which wants to leave the EU.
Cameron's Conservatives trail the opposition Labour Party by about three or four points before the 2015 election, a poll showed this week. UKIP have climbed to third place.
Van Rompuy is the second senior European official to caution Britain over its EU stance this month. Last Wednesday, European Commission President Jose Manuel Barroso accused the Conservatives of copying UKIP and said they would suffer at the European Parliament elections in 2014.
British euroskeptics see the EU as an interfering, expensive and over-powerful bureaucracy that threatens their sovereignty. Some want a new relationship between London and the EU based on trade and others want to leave altogether.
In his speech, Van Rompuy said being an EU member was "not about giving up one's own role" but achieving more by "acting in the world as a team".
"Even in the current economic climate ... Europe is still a commercial superpower," he said. "Pulling apart would weaken our hand and we know it."
Possible trade agreements between the EU and the United States and Japan will hugely benefit all members, he added.
Cameron says it is in Britain's interests to stay inside a reformed EU on new terms, but argues that the European public is disillusioned with the bloc's current state.
Polls suggest the "out" campaign has a narrow lead, although many voters say they are undecided.
(Editing by Alison Williams)