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.
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Fagan-Gayle, a singer song writer who has also appeared on Celebrity Big Brother.
Police were called at around 2.30am on Wednesday and found sledge hammers and pick-axes.
The boy was attacked on a bus in south London. Police say he was stabbed in the stomach.