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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In a wide-ranging Peston on Sunday interview, he said Parliament security needs reviewing and the PM's counter-terror strategy is flawed.
The terror attack carried out by Khalid Masood at Westminster that left four people dead lasted less than two minutes say police.
Two knives were found at the scene in Islington, North London, and the suspects fled on foot before being arrested.