The vehicles have been purchased from the German federal police for the Metropolitan Police, despite Home Secretary Theresa May not having yet approved their use.
Mr Johnson and policing deputy Stephen Greenhalgh sanctioned the purchase following a request from Met Commissioner Sir Bernard Hogan-Howe.
Scotland Yard insists it has no evidence of a specific threat but says the vehicles will plug a gap in its ability to tackle large-scale public disorder.
A majority of Assembly Members oppose the purchase, with many expressing concerns that the vehicles could seriously injure or kill members of the public. Some also worry the Met will break its promise that they’ll be “rarely used and rarely seen”.
Deputy mayor Greenhalgh told AMs that contracts for the purchase have now been signed and that the vehicles would soon arrive in the capital.
He defended the decision to go-ahead before receiving the Home Secretary’s permission to deploy them on the grounds that it was quicker and cheaper to buy three pre-used vehicles. The lead time for new water cannon is said to be around 2 years with an estimated cost of “around £870,000” each.
Mr Greenhalgh insisted the vehicles would be used to police “not protest, but extreme violent disorder”.
He denied suggestions by Green party AM Jenny Jones that the Germans were selling them because of “health and safety concerns”, saying “I’m sure we would not have exchanged contracts if there were health and safety concerns”.
However, when asked what steps he’d taken to assure himself that the vehicles were fit for use in the capital, Greenhalgh sidestepped the question and insisted that was “an operational matter” for the Commissioner.
Questioned about the level of scrutiny he’d applied to the Met’s initial request, the deputy mayor told AMs: “It’s not my job to second guess professional police officers”.
Liberal Democrat AM Caroline Pidgeon warned that Greenhalgh’s push to introduce water cannon into the capital would have “huge” implications for policing across the mainland.
