Water cannons would have been deployed against students during the tuition fee protests in 2010 if the police had access to them, a committee heard today.

Assistant Commissioner Mark Rowley told the London Assembly police and crime committee that Scotland Yard had identified around six occasions when the cannon could have been used in the past 12 years if they had been available.

A public consultation is currently being carried out over plans to try to buy three of the machines from Germany at a cost of £30,000 each, that will then need to be modified for use on British soil.

Critics worry the cannon will be used to quash people's right to protest and demonstrate.

Mayor of London Boris Johnson, who would have to be consulted when the machines were used, said deploying a cannon during the tuition fee protests would have been counter-productive.

He and the Metropolitan Police are aiming to buy three of the machines to have ready for use in the capital this summer, as long as Home Secretary Theresa May agrees to license their use.

Mr Rowley also told the committee that the machines would not have been useful during the rioting that spread across the country in August 2011 - the most serious public unrest the nation has seen in modern times.

He said: "Most of the disorder in the riots, it would not have been a useful tool and we are not presenting it as a silver bullet for all serious disorder."

However the Mayor said that it could have proved effective in Croydon, south London, when the Reeves furniture store was razed to the ground in an arson attack.

Mr Johnson reiterated his desire to see the cannon deployed.

He said: "I think it would be wrong for politicians to say to the police 'absolutely not,' when the police already have access to baton rounds and even more dangerous forms of deterrent."

Other politicians disagree.

Green Party London Assembly member Jenny Jones has made her opposition clear, saying the cannon are "a step in the wrong direction towards arming our police like a military force."