Speaking at the opening of a library at South Ruislip in north west London, Mr Johnson said people could not swear at police and expect to get away with it.
If I read the papers correctly there was a proposal to arrest Mr Mitchell for what he said. That seems to be wholly common-sensical.
The public order act does allow for police officers' discretion in this matter. They've obviously decided not to go ahead with it.
But it shows the gravity of this offence.
More top news
Rob Young has run more than a marathon a day for the last 12 months.
The hospital where the Duchess of Cambridge is due to give birth has had a superbug outbreak.
The London Marathon brings together thousands of people raising money for charity - not to mention some rather unusual costumes.