Boris Johnson is pressing Theresa May to approve his new anti-riot water cannon, in a fresh sign of tensions over law and order policy.

The home secretary has warned she will not be rushed into a decision — even though the mayor has already bought three second-hand machines from Germany and David Cameron has signalled he is happy for police to use them.

The vehicles, which cost £218,000 in June, are currently being tested at a secret location.

The Standard has learned they were disguised as rubbish trucks when they were put on a ferry to the UK in July.

One of the 23-year-old machines broke down and had to be repaired.

They are being tested by the Home Office Centre of Applied Science and Technology, following modifications by Scotland Yard, which formally asked to be allowed to use water cannon in March.

A City Hall source said: “The mayor is keen for them to be approved soon.”

However, in a letter to Parliament on November 20, the home secretary said she would not be hurried into licensing such crowd-control measures for the first time in England and Wales.

Police in Northern Ireland already have several of the vehicles. “The decision on whether to authorise the use of water cannon in England and Wales is a serious one,” wrote May.

“Water cannon, without safeguards, has the capacity to cause harm. It is a tactic that has not been used on mainland Britain previously and there are those who argue its use would change the face of British policing.”

May said she would wait until she could give “full consideration” to all the issues, including medical assessments of potential injuries, which are being carried out by the scientific advisory committee on the medical implications of less lethal weapons. Results are due in the New Year.

The home secretary also wants an evaluation of needs, a list of operational requirements, technical evaluation, and operational performance trials.

A German pensioner, Dietrich Wagner, was blinded and suffered horrific injuries after being hit by a water cannon at a protest in Stuttgart in 2010.

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