The London Assembly announced yesterday that water cannon should not be made available for use against protesters this summer.

Mayor Boris Johnson and the Metropolitan Police had failed to make the case for purchasing three second-hand water cannon, a new report by the London Assembly police and crime committee has concluded.

The Water cannon: Why the Met’s case doesn’t wash report rejected the argument for an “interim solution” before a Home Office decision on the national case for making the weapons available for the first time on mainland Britain.

Despite there being no nationwide consultation on a significant change to the approach to British public order policing, the committee highlighted that the cannon may become available for use by police across the country.

Civil rights group Liberty argued that cannon undermine people’s right to protest.

Liberty policy officer Sara Ogilvie told the committee: “The deterrent effect on those genuine peaceful protesters would actually be quite severe. The thought of things like kettling or getting caught up in violence does put people off.

“If you have a disability or even if you just do not want to get involved in it, it is a real deterrent. It probably would not act as a deterrent to the proper bad, dangerous people.”

Mr Johnson and senior police officers have said water cannon would be “rarely seen and rarely used.”

But uncertainty has arisen over their views on which past disturbances might have warranted their use.

Committee chairwoman Joanne McCartney AM said: “There is no convincing argument for the Mayor’s decision to fund water cannon for the Met for deployment by this summer.

“The Met is pressing for what it calls an ‘interim solution’ without clear justification for its urgency. In doing so, it is preventing a full and proper national public debate about water cannon.

“Professional police officers often press for additional powers and equipment. It is the responsibility of politicians to ensure these are really necessary for the protection of the public, not a creeping infringement of our rights.”