Boris Johnson's decision to buy three water cannon for London has caused tensions at the heart of government, as David Cameron backed the move but the home secretary made it clear she has not approved their use.
The prime minister's official spokesman said he supported their use, while Theresa May made clear she would not be rushed by the London mayor into approving the use of water cannon on the streets of the UK.
May said there were health and safety issues to be considered and denied there had been any delay in their authorisation, pointing out that she only received a formal request for their use from the police in March.
The home secretary's cautious response came as Johnson told LBC that he was prepared to be blasted by a water cannon himself to prove that they were safe.
But less than an hour after May answered reporters' questions, the No 10 spokesman revealed the extent of the disagreement over the issue across Whitehall.
Asked whether Cameron supported water cannon being used, he said: "The prime minister does. There's an issue of principle here in terms of police having the resources they need. They judge, they have submitted an application, that water cannon is one of the resources they believe could contribute effectively, so yes, the prime minister supports that. There's the right processes going on. You have the police and police authorities working with the home office around licensing and the regulatory aspects. It is right that they consider that, but the principle of the police having the resources they need, the prime minister supports that."
Cameron's official spokesman denied that the mayor had jumped the gun by ordering cannon from Germany before the Home Office has approved an application by the police to use them.
The home secretary's comments were her first response to the disclosure that Johnson is to go ahead with the purchase of three second-hand water cannon at a cost of £218,000.
"I will be taking the decision but I want to make sure we are taking it on the right basis. There are difficult issues here. There are health and safety issues that need to be considered," she said after delivering a speech on organised crime in London.
May is understood to have asked for more details on the medical impact of possible injuries from the use of water cannon. Injuries can be caused by flying street furniture hit by the jets as well as by direct blasts from the hoses.
May has already taken advice from scientific experts who specialise in the medical implications of less-lethal weapons but asked for further work to be done clarifying the points they raised. A report is expected to be on the home secretary's desk by the end of this month.
A decision to authorise the use of cannon for the first time would apply to all police forces across England, Scotland and Wales. Already several leading police and crime commissioners, including Tony Lloyd in Greater Manchester, have voiced their opposition.
Lloyd said such a move would fundamentally alter the relationship between the police and the public in the UK. Water cannon have been used in Northern Ireland but their use has not been authorised in the rest of Britain.
The London mayor defended his policing deputy's decision to buy the three water cannon despite the lack of authorisation from the home secretary.
At first he said: "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."
But after being challenged by the LBC presenter, Nick Ferrari, he agreed: "Man or mouse. You've challenged me, so I suppose I'm going to have to do it now. I can see all my press people pulling their hair out over this, but never mind, it's got to be done. Thanks for that one."
In February, a German pensioner told a London meeting that he had been left blinded after being hit by a water cannon. Dietrich Wagner was left unconscious, his eyes irreparably damaged when he took the full force of a water cannon to the face during an environmental protest in Stuttgart.
He said: "Ever since I was hit my life has drastically changed. I can't drive, go shopping, read or do any of the things I used to do. My message is police need to be aware that they are not just a big shower, they are lethal weapons and do serious bodily harm."
His eyelids were torn by the force of the water, damaging the lenses of his eyes and fracturing his orbital bone around the eye.
The document detailing the case for buying the three water cannon contains a plan to limit the losses to London taxpayers if the home secretary refuses permission.
The document says they could be resold and any loss would not exceed £218,000, adding: "The impact of a negative decision from the home secretary can be mitigated by maintaining the option to resell the cannon. There is a proven demand for second-hand water cannon and other European forces have expressed an interest in the devices that we are seeking to buy. "We would seek to take advantage of this to mitigate any losses in the event that no authorisation is granted. Even were it not possible to resell any devices the maximum loss per cannon would be £72,735. This compares extremely favourably to the addition cost of £797,000 should the German devices be sold elsewhere leaving only an option of procuring new devices."
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