Barack Obama is due to arrive in London on Thursday to enter the debate about Britain’s future in Europe, amid fresh warnings that leaving the European Union would jeopardise the special relationship with the United States.

The US president and his wife Michelle will attend a celebratory lunch on Friday to mark the 90th birthday of the Queen, with whom they are said to enjoy a warm rapport, but the president is also expected to add his voice to those calling for Britain to remain in the EU.

With polling suggesting the referendum result will be close, Downing Street has called on the support of a string of global policymakers, from the International Monetary Fund director general, Christine Lagarde, to New Zealand’s prime minister, John Key, to add weight to the remain case. But David Cameron and his allies hope Obama’s star quality will help to win over waverers.

The prime minister told the House of Commons on Wednesday that while, “this decision is a decision for the British people and the British people alone... personally I believe we should listen to advice and I struggle to find the leader of any friendly country who thinks we should leave”.

Nick Clegg, the former Liberal Democrat deputy prime minister, on a visit to the US, said that leaving the EU would plunge Britain into a “new era of isolation”.

Speaking at Princeton University, Clegg said: “Those campaigning for us to leave the European Union like to evoke a sentimental, nostalgic vision of Britannia, proud and independent, ruling the waves once again. But the truth is leaving cannot return us to a halcyon age – if such an age ever existed – and may even mean sacrificing the United Kingdom itself.”

Pro-Brexit campaigners sometimes suggest Britain could strengthen transatlantic ties if its connection to the EU was severed, but Clegg insisted the two relationships were mutually reinforcing.

“If we choose to remain, we will be voting for Britain’s continued leadership position in world affairs; for continued influence in Washington as much as in Brussels or Berlin; and for our reputation as a proud, outward-looking, internationalist power,” he said.

Vote Leave chief executive Matthew Elliott hit back at the former deputy prime minister, saying: “Nick Clegg lied about tuition fees to get himself into government six years ago. He promised a referendum but then backtracked on it. He used to warn we’d disappear off the map if we didn’t join the euro.

“Now he’s telling us it would be a disaster if we vote leave, along with his backers at Goldman Sachs. Voters will be in no mood to be lectured by a politician with more broken promises than there are Lib Dem MPs left in parliament.”

But Clegg’s view was supported by Peter Westmacott, Britain’s ambassador to Washington until January, who writes in the Guardian that, “in America we are seen as more of a force for good, and a more potent ally, inside the EU than outside.”



He adds: “During my time in the US I have found no takers for the fantasy that we can somehow leave Europe and rejoin the world as America’s, or the old Commonwealth’s, best friend and ally.”

Westmacott’s words echoed those of eight former US Treasury secretaries, who served presidents from Richard Nixon onwards, who wrote a letter to Wednesday’s Times warning that the special relationship would be under threat.

Bill Clinton’s Treasury secretary Larry Summers said Brexit would be “most isolationist deed in the last century”.

Obama’s arrival as the referendum campaign is in full swing will be controversial, with former work and pensions secretary Iain Duncan Smith claiming earlier this week that he was only coming, “because the prime minister has gone on bended knee to him and said, ‘come over and help us bully the British people into making a decision’”.

Cameron faces a tough task in reunifying his party once the referendum is over, with almost every day bringing one cabinet minister rubbishing the claims of the opposing camp. Michael Gove, the pro-Brexit justice secretary, used a speech on Tuesday to argue that warnings about the dire consequences of a vote to leave were, “a great, grotesque, patronising and preposterous Peter Mandelsonian conceit”.

It is also ironic as the US president is said to have advised Cameron against holding a referendum on Britain’s membership of the EU according to David Laws, the former Liberal Democrat cabinet minister and author of a new history of the coalition. Laws said that Cameron told Clegg that he knew he was taking a risk with the referendum but was only staging one in order to keep Ukip at bay and retain party unity.

Laws reveals that Clegg formed an alliance with the Americans, including the then US ambassador to London, Louis Susman, to try to persuade Cameron that his increasingly Eurosceptic stance was a profound strategic mistake based on short-term party management. Clegg advised Susman to persuade Obama to tell Cameron directly he was making a mistake, Laws claims.

The ambassador told Clegg the US administration was worried that “you guys may soon not count in Europe any more”. In particular, Laws says Susman expressed his concerns that “David Cameron was not being listened to on Europe, and that the UK was no longer acting as a bridge to Europe for America”.

Hilary Benn, the shadow foreign secretary, said: “The USA is a really important ally and one of our most important strategic and economic partners, so we should take what President Obama has to say seriously. The reason why the leave campaign are trying to attack him is because they fear what he might say about the UK’s place in the EU. The fact that there isn’t a single country wanting us to leave, apart probably from Russia, says a lot about the importance of membership to our future.”

But some leftwing campaigners associate the White House with the controversial Transatlantic Trade and Investment Partnership, known as TTIP, currently being negotiated with the EU, which they fear will strengthen the power of multinational corporations.



Nick Dearden, of pressure group Global Justice Now, said: “There’s a huge contradiction in Obama coming to the UK to support the remain campaign, and Obama coming to Europe to promote TTIP, the toxic trade deal between the EU and the US. This massive corporate power grab being cooked up in Brussels is emblematic of what is wrong with Europe and is a big motivation for people to vote leave in June.”

Labour leader Jeremy Corbyn had hoped to meet the US president, but his team said on Wednesday evening no firm arrangement had yet been made.

• This article was amended on 22 April 2016. An earlier version quoted Nick Dearden describing TTIP as “the toxic trade deal between the EU and the UK”. That was how the quote was supplied to us by Global Justice Now, but the organisation has since made clear that it should have said “the toxic trade deal between the EU and the US”.