There is now "sufficient evidence for a realistic prospect of conviction" of PC Alex MacFarlane, a Crown Prosecution Service review concluded.
Lawyers studied footage appearing to show the officer say to a 21-year-old black man: "The problem with you is you will always be a n*****, yeah?".
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Mayor Sadiq Khan urged Londoners to join him at 6pm in Trafalgar Square.
Pictures of today's events unfolding following a major security incident at the Houses of Parliament.