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
Residents who live near Epping Forest are complaining after a private hunting club was given a licence to shoot deer in the forest.
Police have launched an appeal for help to find a 12-year-old girl missing from Lewisham.
Christmas shoppers have hit central London for the first Sunday in December.