BARACK Obama will today make the most significant intervention of any foreign leader in the UK’s in-out referendum campaign when he will come out strongly in favour of Britain’s membership of the EU.

As with his statement during the Scottish independence referendum when the US President said he hoped the UK would remain “strong, robust and united,” David Cameron is confident that Mr Obama will give a forceful endorsement of the Remain camp and hopeful it will help swing undecided voters in favour of In.

Before the president was due to arrive in London last night, his administration made clear he would back the Prime Minister.

Ben Rhodes, his deputy national security adviser, told reporters Mr Obama would make clear the decision was for the British people but, if asked his view, would give it and that would mean "being very straightforward and candid as a friend as to why the United States believes that it is good for the United Kingdom to remain in the European Union".

However, Mr Obama’s intervention – due at a Downing St press conference this afternoon - has already been dismissed by the Leave camp. Nigel Farage, the Ukip leader, urged the president to “butt out” and branded him the “most anti-British president” in history.

Today, Vote Leave accused him of preparing to “jump aboard the Government’s campaign to intimidate the British people into voting to stay in the EU” and of peddling “double standards”.

Outer Iain Duncan Smith echoed the point by saying: “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.”

The former Work and Pensions Secretary said he could imagine no circumstances under which the president would lobby for the US Supreme Court to be bound by the judgements of a foreign court or accept laws should be made for or taxes imposed on American citizens without the approval of Congress.

“If he believes it would be unacceptable for the American people - those he is actually elected to guide - I fail to see how it is appropriate for him to recommend that the British people continue to relinquish their right to democratic self-governance,” said Mr Duncan Smith.

Fellow Brexiteer, London mayor Boris Johnson, claimed it would be "hypocritical for America to urge us to sacrifice control - of our laws, our sovereignty, our money and our democracy - when they would not dream of ever doing the same”.

But Nick Clegg, the pro-EU former Deputy Prime Minister, used a speech at the prestigious Princeton University on Wednesday to urge Mr Obama to make the case for a Remain vote "loudly and clearly" during his visit, warning that Brexit would not only lead to an end to the UK-US special relationship but also the break-up of Britain.

During the two-day visit - his fifth to the UK while president - Mr Obama will also have lunch with the Queen today and hold a town hall-style meeting with young Londoners.

While his No 10 bilateral with the Prime Minister this afternoon will be ostensibly about fighting the threat from so-called Islamic state, it will be his intervention into the EU debate that will gain most media publicity and create headlines.

Ahead of the president’s arrival, Mr Cameron said: “Britain’s relationship with the United States is special and enduring. Based on shared values and convictions it has stood the test of time. I am deeply proud of what it has allowed us to achieve, in dealing with the global challenges we both face and ensuring the security and prosperity of our people.”

He said such challenges were more urgent today than ever before with IS’s brutal campaign of terror that was causing instability and unimaginable suffering in Syria, Iraq and beyond.

The PM added: “I am confident that Britain and the US can continue to build on a solid basis of friendship and a shared commitment to freedom, democracy and enterprise to shape a better world for future generations.”

Meantime, the Commons Scottish Affairs committee has announced it is to consider the impact of what staying in and leaving the EU would have on Scotland and also the consequences of different parts of the UK voting in different ways.