Boris Johnson's office has approved the purchase of three water cannon from the German Federal Police - despite their use not yet being authorised by the government.
Deputy mayor for policing and crime Stephen Greenhalgh signed off on the deal this week before the Home Office has confirmed their use will be allowed in England and Wales. They have not previously been used on the British mainland.
In a report published after consultation on the matter, Mr Greenhalgh said the terms of the deal agreed with Germany meant City Hall could save £2.3m on the cost of buying new devices.
The offer from the German police only stood until the end of July, the report said.
Mr Greenhalgh acknowledged: "It is open to the Home Secretary to make a decision about whether or not to authorise the use of water cannon on the UK mainland."
He added: "We do not expect further such opportunities to purchase at such an advantageous cost."
The total cost for the three cannon will be £218,000, including the purchase and expenses associated with their servicing and transportation.
A Home Office spokesman said: "We are keen to ensure that the police have the tools and powers they need to maintain order on our streets."
Labour's only declared mayoral candidate, Christian Wolmar, said the Mayor had made a mistake in authorising the purchase.
"Boris gets it wrong on water cannon - I hope they'll never be used on London streets. They are totally inappropriate method of crowd control," he tweeted.
