Boris Johnson said he was "very nervous" about playing Pippa Middleton at ping-pong after she challenged him to a 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?"
Mr Johnson spoke at Ealing Studios, home to the Downton Abbey set, after announcing a £750,000 investment in London's TV and film industry.
The investment, which aims to create 1,000 jobs and bring in £200 million worth of expenditure, comes after a government-announced tax relief for film. The money will be invested in TV and TV animation through Film London, an agency that helps UK productions, along with this year's existing £1.3 million budget.
Mr Johnson said: "We want to make sure we're not losing money to places like Hungary or Ireland where they are making TV shows that could well be shot in London. London is now one of the biggest filming cities in the world and we are now going to see the same thing with TV and games-making as well."
Lord Hall, director-general of the BBC, said: "These are certainly exciting times for television production. The capital is already one of the most TV-friendly cities in the world and we look forward to continuing to play our part in this vital part of the British broadcasting ecology."
Julian Fellowes, creator of Downton Abbey, said: "It is encouraging to feel that the tremendous, worldwide success of British high-end television is at last receiving some recognition and help."