The truck-based cannon are currently banned on the UK mainland and any purchase would have to be approved by Home Secretary Theresa May.
In a letter to Mrs May, the Mayor says if the purchase is approved his office would consult the public to “confirm the support in London for the use of water cannon in the most extreme circumstances” before making a final decision to buy one.
The Mayor’s letter says Met Commissioner Sir Bernard Hogan-Howe has indicated the cannon “would be rarely seen and rarely used”.
It also states that Mr Johnson would ask the capital’s new policing ethics panel to undertake a study of how the cannon should be deployed and used.
In a separate letter, Deputy Mayor for Policing Stephen Greenhalgh has told London Assembly members that the cannon is wanted by the summer.
London Assembly Member Baroness Jenny Jones says introducing water cannon in the capital would be “a step in the wrong direction towards arming our police like a military force”.
She suggested there would be conflict between the Mayor and Commissioner on when to deploy the cannon and said Londoners needed to know “when and in what circumstances the Mayor would agree with the Met using this weapon.”
She also voiced concern that the Met could use the cannon “against people exercising their legal right to protest.”
