Charlotte Edwardes, a columnist, wrote in The Sunday Times that Johnson had groped her at a lunch in 1999 when he was editor of The Spectator magazine.
Asked in a television interview if he had inappropriately touched her thigh as she reported, Johnson said: "No".
When subsequently asked if she had made up the account, he replied: "I'm just saying what I said, and I think what the public want to hear is what we are doing for them and the country and for investment in ways of uniting the country."