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(Corrects headline to London's, not Britain's, mayor)
LONDON, Dec 4 (Reuters) - London Mayor Boris Johnson said on Tuesday he would campaign to keep Britain within the European Union if the country renegotiated its relationship to be based around the single market.
"I always came down in favour, narrowly, of staying," he told a Thomson Reuters Newsmaker event in London, when asked whether he would campaign for Britain to stay within the single market. (Reporting by Kate Holton; editing by Guy Faulconbridge)