The Mail on Sunday has quoted a source suggesting that strategist Lynton Crosby told Boris Johnson to concentrate on traditional Tory voters instead of 'f****** Muslims'.
A spokesman for Mr Crosby told the newspaper that he had "absolutely no recollection" of using the phrase and said it was "mischievous" to suggest he was racist "in any way".
Johnson said: "I don't recognise this characterisation. He's the best campaign manager I've ever seen. The Conservative Party has done well to retain his services."
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Police in New York are still investigating what the substance is and whether it could be harmful.
The Sherlock actor made the comment while he received an award for his humanitarian work.