BORIS JOHNSON was humiliated yesterday by Home Secretary Theresa May in rejecting the use of water cannon — after he splashed out more than £200,000 on three.

The London mayor awkwardly sat behind her in the Commons while she refused a formal police request to deploy them, arguing they would cause serious harm and damage to public trust.

Mr Johnson wasted taxpayers’ cash on the faulty 25-year-old German cast-offs despite the weapons in other countries blinding protesters and causing severe spinal injuries.

In June last year, he told the London Assembly that it was “most unlikely” the application would be refused, but now he insists on keeping them “in reserve” and training police to operate them for crowd-control, even though the practice is illegal.

Ms May could be trying to undermine the mayor before their contest for Conservative Party leadership, critics have said.

In a statement, the Metropolitan Police said they are “naturally disappointed” with the decision.

Water cannon have never been used in Britain but they have been deployed in Northern Ireland.