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
Police say they have traced four drivers who are thought to have hit the woman, and added that a fifth vehicle may also have been involved.
Police are trying to trace two men after two homophobic attacks on gay men in Walthamstow
Two London Boroughs make it into the top 15 places to live in the UK.