A 46-year-old female police officer from Scotland Yard's Diplomatic Protection Group was arrested over alleged leaks to the media linked to the Andrew Mitchell "plebgate" affair, the Metropolitan Police said.
A 46-year-old male officer arrested yesterday has been bailed to return on a date in March.
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MPs say there must be no way back for the former London mayor as he hints he could seek to rejoin the party.
President Donald Trump has requested that the Justice Department investigates whether the FBI infiltrated his campaign.
High pressure will help keep conditions relatively quiet through the course of this week.