Boris Johnson said he was "very nervous" about playing Pippa Middleton at ping-pong after she challenged him to a "whiff-whaff" match.
The Mayor of London accepted Ms Middleton's offer on Twitter after she said she would like to see whether the Middleton or Johnson family was the most competitive.
The mayor said: "I feel very nervous clearly because she seems to be very good. Well, she claims to be very good.
"But I'm very happy to do this because it's a chance to promote what we're doing with sports legacy London.
"We have greatly increased the number of people doing sports. If Pippa's kind offer can be used to promote that then I'm all for it."
He then seemed to confuse Ms Middleton with her sister the Duchess of Cambridge, and asked himself: "Pippa Middleton. She's a duchess is she? Or isn't she just a sister of a duchess?"
Johnson spoke at Ealing Studios, home to the Downton Abbey set, today after announcing a £750,000 investment in London's TV and film industry.
At the studios, Johnson attempted directing an animation scene in the special effects studio where Gollum from The Lord Of The Rings was created.
He joked: "Would I be interested in being in a film? You betcha. But no one's curiously made me an offer. I think the movie industry is moving along without me at the moment.
"Who would I have to play me? You mean, apart from Brad Pitt obviously."