The Mayor has asked Home Secretary Theresa May for permission to purchase three German water cannon for the Metropolitan Police and is currently consulting Londoners on their deployment.
Earlier this month Mr Johnson said Met Commissioner Sir Bernard Hogan-Howe believed they would “rarely used and rarely seen”.
Questioned by the London Assembly’s Police and Crime Committee, the Mayor said he would be wrong to deny the force equipment which can help it protect lives or property.
He suggested questions would be asked of him if a future incident occurred in which the cannon could have aided the police restore order and he had denied the Met’s request.
Green party Assembly Member Baroness Jenny Jones expressed concern that the cannon would be used against protestors.
However Assistant Commissioner Mark Rowley said the force “would never use them against protests” and that they would be held back for extreme circumstances.
Conservative AM Victoria Borwick said purchasing the cannon risked contradicting the Mayor’s message that crime was falling and London was getting safer.
In response Johnson insisted it was “vanishingly rare that such eventualities will occur” where the cannon would be deployed and said the Commissioner had agreed they would not be deployed without political consultation.
Asked what would happen if the Mayor opposed their use, Rowley said he would make the decision, taking account of any view the Mayor expressed.
Rowley said public feedback suggests Londoners are concerned the Met lacked the “assertiveness” to respond to “rare” major incidents.
He added that they were a “credible tactic” against “breakaway factions” intent on causing harm such as the group who attacked the Millbank Tower during the student demonstration.
However AM Borwick said such groups would not be deterred by “lumbering vehicles which…are emptied in five minutes”.
Despite Rowley’s insistence that the cannon would be useful, he appeared to concede that they were slow and could not always be moved to trouble spots in time to assist officers.
Johnson said he accepted “water cannon would not have made a blind bit of difference in Tottenham” when residents protested at the police’s shooting of Mark Duggan but suggested they may have been of use when unrest expanded to Croydon.
Labour’s Jennette Arnold questioned the value of the purchase which are understood to be just 2-3 years from their end of service date and would possibly need to be replaced without ever having been used.
Despite the scepticism of many of the committee, the Mayor’s stance was backed by Tony Arbour AM who said the Mayor “should not withhold” the cannon from the Met.
The committee will question Stephen Greenhalgh, Deputy Mayor for policing and crime, and senior Met figures about the need for the cannon at a further meeting on Thursday.