David Cameron will meet US presidential candidate Mitt Romney at Downing Street today, during Romney's visit to the capital.
The Republican candidate is using the trip to raise campaign funds and canvass for support among London's large American community.
Romney, who is trailing Democrat president Barack Obama by a narrow margin in the latest polls, is also due to meet Deputy Prime Minister Nick Clegg and Labour Leader Ed Miliband.
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The anonymous artist said the prank had worked “every time” in rehearsals.