Water cannon are often used during riot situations in Northern Ireland, but have never been used in Britain where they would first need clearance from Home Secretary Theresa May.But Mr Johnson has already agreed to spend £130,000 on three second-hand vehicles from the German police, which would apparently have been sold to another buyer if the deal was not completed by July.Challenged by LBC interviewer Nick Ferrari to be filmed getting soaked by water cannon, Mr Johnson came round to the idea after some consideration."I'm certainly prepared to do anything to show that they're safe within reason. I'm not quite sure whether I want to stand in front of a water cannon - I haven't done anything to deserve it," he said."I'm doing this unfortunately from off-base, so I can see all my poor press people are going to be tearing their hair out. Never mind, it's got to be done."Okay. Man or mouse. Alright, you've challenged me to this. I suppose I'm going to have to do it now.London Mayor Boris JohnsonMr Johnson has denied that he is "playing politics" by agreeing the purchase without the Home Secretary's approval.Theresa May has indicated that her decision on water cannon will not be influenced by the development, insisting it was be "taken on the right basis".She added: "There are health and safety issues that need to be looked at very carefully."However Mr Johnson has said he believes it is "highly likely" that approval will be granted.Even if water cannon are licensed for use in England and Wales, an independent ethics panel will look at how and when they should be used.Labour and the Liberal Democrats have opposed Mr Johnson's decision to purchase the vehicles, claiming there is not enough evidence that their use is effective in maintaining order.A spokesman for David Cameron said the Conservative Prime minister supports the idea in principle."There is an issue of principle here in terms of the police having the resources that they need, and if they judge - as they have - that water cannon is one of the resources that they believe can contribute to effective policing, the Prime Minister supports that," he said.Calls were made for Northern Ireland's six water cannon to be deployed to London during the serious riots in 2011, but those were met with opposition from some quarters.An emergency meeting of the Cobra committee did see contingency plans put in place which effectively placed the water cannon on 24-hour notice, but they were not deemed to be required.