The Met is planning to buy three second hand water cannon from German police at an estimated cost of £200,000.
Speaking this morning, Boris Johnson said: "I have concluded there is broad support amongst Londoners for the use of this measure by the police in limited circumstances."
The majority of the 25 Assembly members - including Conservatives - are opposed to water cannon.
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A teenager who was mistakenly released from prison after he was arrested on suspicion of attempted murder has been detained by police.
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