Boris Mayor 4 Cardiff

He said: "I would advise that the President has the right wherever he is to explain what is in the interests of the United States of America. And since the US is our one indispensable ally, our biggest single trading partner and the ultimate guarantor of our security, its interests matter to everyone in Britain whether we like it or not."

He added: "A further reason why the President should feel free to speak out is that the last thing America, or the West in general, needs over the next few years is a self-inflicted bout of introversion and instability. The financial disruption of a British exit would be enough on its own to worry Washington, but their biggest concern would be the years of effort diverted by their main allies into a long and very painful attempt to negotiate a new relationship."

Writing in the Evening Standard, former editor of The Times, Bronwen Maddox, said "there is no doubt that he [Obama] knows the potential impact of his “very candid” message that the US wants Britain to remain in the EU."

Ms Maddox said: "Obama bears a real gift, perhaps the most valuable he could give Cameron. His endorsement for Britain’s membership of the EU, I think, is a powerful shot for the 'Remain' camp. Sure, there are those who say it will backfire, causing people to retort that foreign leaders should mind their own business."

She added: "... following an equally explicit 'Remain' message from the International Monetary Fund, Obama’s statement will impress on many that Britain’s international partners really don’t want us to go it alone."