The mayor of London has been criticised for pedalling his barrister wife Marina Wheeler through the capital on Thursday on a bicycle designed for one person.
Giving a backie is illegal under the terms of the 1998 Road and Traffic Act. Offenders can be fined up to Â£200.
The mayor has apologised, saying he did not know he was breaking the law.
Sam Jones, from cycling charity CTC, said his actions were "very naughty".
Mr Jones said: "We wouldn't encourage other cyclists to carry passengers in such a fashion. We would never encourage cyclists to break the law."
The footage of Mr Johnson and his wife, obtained by the Sun newspaper, was shot by passengers in a passing car as the couple cycled along a road in North Kensington.
It shows Ms Wheeler sitting on the saddle, without a helmet, holding her handbag as her husband stands on the pedals.
After the passengers informed the mayor he was breaking the law, Mr Johnson stepped off the bike and said: "Night, night," to them.
They can be heard in the video asking the mayor: "Mate, you all right? Saddling, that's a good one, eh?" Another says: "You're not allowed to do that, mate."
Section 24 of the Road and Traffic Act states: "Not more than one person may be carried on a road on a bicycle not propelled by mechanical power unless it is constructed or adapted for the carriage of more than one person."
The mayor, whose responsibilities include London transport, is a keen cyclist and cycles available for public hire in London have gained the nickname "Boris bikes".
Mr Johnson's official spokesman said: "The mayor wishes to apologise for offering his wife a short-lived lift on the back of his bike!
"He was unaware that he was apparently in contravention of the Road and Traffic Act. He wasn't intending to ride all the way home from North Kensington to North London with Marina on the back; rather he was attempting to transport his wife to a main road, from where they hailed a black cab for her.
"As everyone knows the mayor is a huge supporter of cycling, and an even bigger fan of the black cab trade, hence his desire to combine the two!"
Boris Johnson "should have known better" when he gave his wife a "backie" on his bike while cycling in London, safety campaigners have said.