Obama's arrival in UK brings a boost for the Remain campaign

The president’s advisors have indicated he will ignore criticism from some Leave figures that he is interfering in British affairs and offer his support to the Prime Minister on the issue.

President Obama is due to arrive in the UK tonight and will take part in a press conference with Mr Cameron tomorrow as well as having lunch with the Queen.

A popular figure in the UK, the president’s endorsement will be enthusiastically welcomed by Remain supporters.

Former deputy Prime Minister Nick Clegg today welcomed president Obama’s support for Britain’s EU membership and argued a vote to leave would also damage this country’s relationship with the United States.

The Sheffield Hallam MP said: “The stronger both relationships are, the stronger Britain is. But if one is weakened, they both are weakened.

“When Britain votes on whether to remain in the European Union, it will have a profound impact on Britain’s standing in the world for decades.

“A vote to remain is for Britain’s continued leadership in world affairs, influence in Washington (as much as in Brussels or Berlin) and for our reputation as a proud, outward-looking, internationalist power.

“But a vote to leave is to diminish that position. We will reduce our relationship with our neighbours, making ourselves less influential, and therefore less valuable as an ally to our American cousins. We will sacrifice our seat at the top table and our proud internationalist heritage for isolation.

“Those campaigning to leave evoke a nostalgic vision of Britannia, proud and independent, ruling the waves once again.

“But leaving cannot return us to a halcyon age – if it ever existed – and may mean sacrificing the United Kingdom: leaving is likely to trigger a second independence referendum in Scotland and possibly the break-up of the United Kingdom.”

Former Downing Street communications director Alastair Campbell claimed a British exit from the European Union would play into the hands of Islamic extremists.

In a speech in Dublin, he said: “No serious overseas player thinks we should leave, unless it suits their agenda.

“Putin probably. Isis definitely. And, depending what mood he is in, Donald Trump.

“That’s about it on the international scene.”

The former adviser to Tony Blair said the referendum was of greater significance than any of the general elections in which he had been involved.

“The consequences - for jobs, living standards, culture, national security and our standing in the world - are greater,” he said.

Mr Campbell added “if I had to put my life on it, right here, right now” he believed the UK would vote to stay within the EU.

Earlier this week the Treasury published analysis suggesting Britain’s departure from the European Union would cost households on average £4,300 a year.

The document has been widely derided by Leaver campaigners but experts from the London School of Economics yesterday argued that, if anything, the Treasury had been too cautious in its estimates.

The Treasury forecast suggests Britain’s GDP would be cut by 6.2 per cent but economists at the LSE’s Centre for Economic Performance put the figure at between 6.3 and 9.5 per cent.

Swati Dhingra, from the LSE, said: “‘The Treasury Report looks at the realistic options the UK will face after Brexit - and the cost of each. It takes a conservative approach to the potential costs.”