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Boris Johnson today laid into Barack Obama for refusing to pay London’s Congestion Charge while telling Britons to hand over their hard-earned money to the European Union.

The Mayor hit out as exclusive Ipsos MORI research for the Evening Standard revealed the American president could sway one in five wavering voters in the knife-edge EU referendum — potentially enough to tilt the balance on June 23.

Mr Obama was making an impassioned appeal for Britain to stay in the EU this afternoon after talks at No 10 and lunch with the Queen.

But Mr Johnson called him “downright hypocritical”, telling the Standard: “It is always good to hear from our friends in America but Londoners will know that the United States guards its sovereignty with such hysterical jealousy that US diplomats still refuse to pay the Congestion Charge in this city.

"They owe us more than £9 million that could go on transport improvements.

“Who are they to tell us to send £350 million every week to Brussels?”

American diplomats in London — and the presidential vehicle, dubbed The Beast — refuse to pay the C-charge.

The Mayor said Mr Obama would never dream of giving away America’s freedoms in the way that he wanted Britain to surrender sovereignty to EU institutions.

“The US has refused to sign up even to the International Criminal Court or the UN Convention on the Rights of the Child,” he said.

“Who are they to say the UK should continue to be ever more deeply enmeshed in a federal EU where 60 per cent of our law already comes from Brussels?

"We will prosper and thrive outside the EU as never before and we will be better and stronger allies of America.”

Ipsos MORI found Britons are split down the middle over whether Mr Obama should tell them how to vote, with 49 per cent welcoming his intervention and 46 per cent saying he should stay out of it.

Overall, 62 per cent of the public say the president’s view is “not at all important” when it comes to making up their minds.

But the research found he could motivate significant numbers of people in the most crucial group of voters — those who say they will vote for one side but admit they may change their mind before June 23.

Some 21 per cent of them thought Mr Obama’s message was important to their vote. That included a quarter of soft Remain voters, and 17 per cent of wavering Brexit backers.

Mr Obama set off with wife Michelle to lunch with the Queen and Prince Philip at Windsor Castle this morning.

Gideon Skinner, head of political research at Ipsos MORI said: “President Obama may not change the minds of many Leave supporters — indeed they want him to stay out of the debate.

"But he could play a bigger role in bolstering the views of those already leaning towards Remain.”