Three water cannon bought for the UK’s largest police force are to be sold off without ever being used, having spent two years hidden away in Kent.

Former London mayor Boris Johnson authorised the purchase of the second hand trucks from Germany, at a cost of more than £200,000, following riots in the capital in summer 2011.

Home Secretary Theresa May — the early favourite to succeed David Cameron as Prime Minister — banned them from use and they were subsequently stored at a Met training facility in Gravesend.

Mr Johnson’s successor Sadiq Khan said the money raised from their sale will be used to fund youth services.

Speaking during his first State of London debate, as reported by the Evening Standard, Mr Khan said: “One of the deals Boris Johnson managed to do with the Germans was to buy three water cannon. What I’m going to do is sell them and use the money for youth services.

“This shows the inability of Boris Johnson to get a good deal. Second hand, paid almost £250,000. We’re paying for storage facilities for these water cannon. That beggars belief.”

The home secretary also teased Mr Johnson over his deal for the water cannon as she announced her intention to run for leadership of the Conservatives.

She said: “I seem to remember last time Boris did a deal with the Germans he came back with three nearly-new water cannon.”

She banned them from use in 2015 because of the risk of injuring people they were used against.