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Boris Johnson ’s three illegal water cannons, costing taxpayers hundreds of thousands of pounds, are gathering dust after failing to find a buyer.

The cannons have now been on sale for more than a year via the Ministry of Defence .

They were bought in 2014 when Boris was mayor of London but immediately banned from the streets by then-Home Secretary Theresa May , on the basis that UK police should never use military-style equipment, and have never been fired in public.

Boris ordered them following the 2011 riots as he promised to “get medieval” on troublemakers.

(Image: PA)

Current Mayor Sadiq Khan put the useless cannons up for sale when he took up office in 2016, promising to use the cash to fund youth projects and tackle gang crime.

But they are still being stored in a secure Metropolitan Police compound.

Boris, now Foreign Secretary, paid £218,000 to the German Federal Police for vehicles, which were 25-years-old at the time of the sale, but maintenance and modifications to the second-hand trucks, which were found to have 67 faults at the time, have seen costs spiral to more than £320,000.

(Image: PA)

In 2015, Theresa May told MPs that her decision to ban their use followed exhaustive medical and scientific tests which led her to conclude that the cannon had the potential to cause serious injuries including spinal fractures, concussion and eye injuries.

Mrs May also raised concerns over their usefulness in fast-moving riots, and that they were likely to damage the British tradition of policing by consent.

It is hoped they would sell for up to £43,000 each but if a buyer is not found, it is feared they could go for scrap value, as little as £2,000.

(Image: PA)

Mr Khan said: “The cannon are currently still for sale. The MOD is helping to find the highest and most reputable bid from a European policing or civil protection organisation – according to conditions set by the German Federal Police [at purchase].

“We have been left in this position by the previous Mayor who rashly purchased them before he even had permission to use them.

“I am now attempting to claw back as much of London’s taxpayers’ money as I can.”

Sian Berry , from London’s Green Party , said: “They’ve only ever been used three times in training exercises.

(Image: REX/Shutterstock)

“On top of the cost of buying the cannon, money has been wasted training people to use them.

“We are still clearing up Boris’ mess and will be doing so for a long time to come.”

Other projects launched by Boris when he was mayor have been criticised for blowing huge amounts of cash.

A planned Garden Bridge over the Thames swallowed £40m in public cash before it was scrapped last year; the most expensive cable car ever built (£60m) has just four passengers a day, a report found; and a new series of ‘Boris buses’ got so hot inside it was described as “cauldron on wheels”.