The Guardian view on London's water cannon

http://www.theguardian.com/commentisfree/2014/jun/13/debatable-decision-water-canon

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Government has a duty to maintain law and order. It also has a duty to protect the right of free assembly, including the right of protesters to demonstrate peacefully, subject to proportionate policing. The hurried purchase from Germany of three secondhand water cannon by London's mayor, Boris Johnson, should prompt urgent debate about the balance between the two.

His decision was supported this week by David Cameron, even though Mr Johnson had preempted the considered view of the home secretary, Theresa May. The mayor says he was forced to act quickly to secure the water cannon at a competitive price. But the sequence of events is troubling. For the embrace of water cannon by the largest and most influential force in the country is not a municipal issue. It is a matter of national importance, not least because London's equipment will be regarded by some police chiefs as a "national asset". Water cannon have been deployed in Northern Ireland – the province has six – but the first deployment on the streets of the capital will immediately change the face of policing and alter how Britain itself, not just London, is seen around the world.

Consider the hardware itself. The 30-tonne Ziegler WaWe 9000 water cannon can fire up to 18 litres of water a second. It is classified as a "less lethal" weapon, as opposed to non-lethal, because water cannon can kill. This model isn't even state of the art. Though the mayor has offered to reassure the public by being cannoned himself, Germany is phasing out this model out on safety grounds. Then there are the practical difficulties. Water cannon would have had little effect on suppressing the riots in towns and cities across the country in 2011. They are most effective when straight lines of police face similar lines of demonstrators. This may be why the enthusiasm of many chief constables is muted. They may also be concerned about repercussions if the use of the equipment is challenged under the human rights act.

The mayor and Sir Bernard Hogan-Howe, the commissioner of the Metropolitan police, have clearly satisfied themselves of the need for further firepower. No one doubts that they have the right to make such a decision; we support devolved government in the capital. But it's worth noting that the London assembly voted against the acquisition of water cannon. One dissenter was the mayor's last policing adviser. Another was his own deputy.

Westminster should have a proper say, and it could do so without offending constitutional proprieties. The mayor will have his water cannon: the home secretary can ensure they never leaves the depot. She should do just that.