Barack Obama has thrown his support behind the campaign to keep the UK in the EU, arguing Britain's influence on the world stage is magnified by its membership of the 28-nation-bloc.
The outgoing US president's intervention coincides with his visit to the UK, where he will meet with The Queen to celebrate the monarch's 90th Birthday and hold talks with Prime Minister David Cameron.
Obama, pre-empting attacks from pro-Brexit campaigners, argued he had a right to back a 'remain' vote because of the ultimate sacrifice tens of thousands US soldiers made on European soil during World War II.
"Our special relationship was forged as we spilt blood together on the battlefield. It was fortified as we built and sustained the architecture for advancing stability and prosperity in Europe, and our democratic values around the globe," he wrote in The Daily Telegraph.
The president also urged UK voters to take stock of the country's relationship with the EU, with just over two months to go before the 23 June ballot. "The EU doesn't moderate British influence – it magnifies it. A strong Europe is not a threat to Britain's global leadership; it enhances Britain's global leadership," Obama argued.
"The US sees how your powerful voice in Europe ensures that Europe takes a strong stance in the world, and keeps the EU open, outward looking, and closely linked to its allies on the other side of the Atlantic. So the US and the world need your outsized influence to continue – including within Europe."
Alan Johnson, the chair of Labour in For Britain, seized on Obama's remarks and defended the president's decision to intervene in the referendum.
"President Obama is Head of State in a country that has been Britain's ally in war and in peace," the former home secretary declared.
"US soldiers lost their lives in two world wars on our continent. Not only does this give the President an entitlement to comment, I believe he has an obligation to point out the wider ramifications of a British withdrawal from the EU."
But the Vote Leave campaign accused Obama of "double standards". Iain Duncan Smith, the former work and pensions secretary, claimed the president would never accept the US being a member of political or economic union similar to the EU.
"President Obama, and every one of his predecessors, have ferociously protected the sovereignty of the USA – and I only wish we could say the same of our leaders," Duncan Smith said.
"What I do find strange is that he is asking the British people to accept a situation that he patently would not recommend to the American population.
"I can imagine no circumstances under which he would lobby for the US Supreme Court to be bound by the judgements of a foreign court. Nor can I imagine any circumstances in which he would accept that laws should be made for – or taxes imposed on – the people of the US without the approval of Congress."
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