The leading campaigner for Britain to leave the European Union gave the secret away in a comments to the Sunday Times.
"This is the real thing," the paper quoted Johnson as saying. "It's all natural". But when asked whether he dyed it, the 51-year-old added: "Yes".
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Johnson's shaggy, unruly hair has helped make him one of Britain's most recognisable and popular politicians, along with his bumbling manner and love of witty soundbites.
The Conservative lawmaker split with Prime Minister David Cameron by announcing in February that he would campaign for Britain to leave the EU in the June 23 referendum.
He is one of the favourites to succeed Cameron if the premier steps down following the vote, which looks like being a close fight between "Leave" and "Remain".
A Sunday Times/YouGov poll this week found that 20 percent of those surveyed wanted Johnson to take over from Cameron.
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It put him well ahead of his nearest rival, Home Secretary Theresa May, on eight percent.
That is despite Johnson once insisting that his chances of becoming prime minister were only slightly better than those of him "being reincarnated as an olive".
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