Plans to unleash a dangerous and indiscriminate riot control weapon on the streets of Islington have been panned by critics.

Mayor of London Boris Johnson and the Met Police are considering splashing out £200,000 on three second hand German water cannons to use the worst cases of large-scale public disorder – they say the weapons would be “rarely used and rarely seen”.

But Islington’s opposition Lib Dem group have written to City Hall objecting to the scheme – they say the cannons, which shoot out 15 litres per second, run the real risk of injury to protestors and could inflame otherwise peaceful situations.

Cllr Terry Stacy, Lib Dem leader, said: “These plans by the mayor to put water cannon on the streets of the capital – and potentially Islington – fly in the face of common sense and could, in fact, provoke more violent reactions from people exercising their right to protest, as well as injury.

“The fact that the Mayor thinks that spending all this money on water cannons is a good use of public money shows how he does not live in the real world sometimes.

“When so many Islington residents are struggling financially and have seen police stations close locally and police numbers fall, they will ask what on earth the Mayor’s priorities are for keeping our streets safer.

”We are totally opposed to the Mayor’s plans for water cannons on the city’s streets.”

Cllr Paul Convery, Islington Council’s executive member of community safety, said: “We’ve written to City hall to oppose the idea.

“They’re just not very practical amongst other things.

“In the sort of disorder it might be used against, like the Riots in 2011, you don’t know where it’s going to kick off. These are big, very heavy vehicles that take a long time to move about.

“And there is no doubt if you get hit full force by one of these things it can do you serious harm.

“The Met has got very good at crowd control methods over the years and they don’t need to bring water cannons in – it’s selling their skills short.”

Stephen Greenhalgh, deputy mayor for policing and crime, said: “Surveys have indicated strong support for the police to have every tool at their disposal to prevent riots.

“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. The Mayor has given his support in principle, subject to proper rules for deployment, but our tradition of policing by consent means we do not want to go ahead until we have heard from Londoners.”

Later this month the Mayor will make the decision whether to ask the Home Secretary Theresa May for permission to water cannons in the UK.