Police use of high-powered water cannon against protesters in London loomed closer yesterday as a six-week public consultation on the idea opened.

The Metropolitan Police Service says the cannon would only be used in response to riots and other serious and exceptional public order events. But critics worry they could be used to crush freedom to protest.

British police use of such cannon is currently restricted to Northern Ireland.

However, since the summer riots of 2011 Metropolitan Police Commissioner Sir Bernard Hogan-Howe has repeatedly backed water cannon as public order tools in certain situations and has asked London Mayor Boris Johnson to fund the purchase.

The city's deputy mayor for policing and crime Stephen Greenhalgh said: "The professional view of police leaders is that water cannon would be a useful tactic to help protect people and property in response to extreme public disorder.

"This engagement period will allow the police to discuss why they want water cannon, explain how and when they might be used and to address any concerns people may have.

"Once we have heard from Londoners we will consider the responses and share these with the Home Secretary, who has to make the final decision."

But Green Party London Assembly Member Jenny Jones was tore into the plans.

She said: "Would the mayor have supported the deployment of water cannon against students protesting against their fees going up? What happens if the commissioner wants to deploy water cannon but the mayor doesn't?

"Londoners need to know when and in what circumstances the mayor would agree with the Met using this weapon.

"Allowing water cannon on the streets of London is a step in the wrong direction towards arming our police like a military force, and it goes against our great tradition of an unarmed police service.

"People have a democratic right to protest and my fear is that once the mayor allows these weapons onto our streets we will see them being used against people exercising their legal right to protest."