Boris Johnson’s mayoral administration spent £1,000 fitting stereos and CD players to armoured water cannons purchased by the then London mayor, it has emerged.

Details of the mounting costs of the units, which will never be used in the UK, were made public by the office of Mr Johnson’s successor Sadiq Khan on Wednesday.

Mr Johnson bought the cannons second hand from the German police in 2014 while he was mayor of the capital, but was blocked from using them by Theresa May, who was at the time Home Secretary.

The Greater London Authority (GLA) now hopes to recoup at least £35,000 by selling the cannons on again. In total the cannons cost taxpayers £323,000, according to figures released by the GLA.

Mr Khan said: “It beggars belief that such a huge amount of taxpayers‘ money has been wasted on paying to store these redundant machines.

“We’ve been left in this position by the previous mayor who rashly purchased them before he even had permission to use them, and now it’s my job to claw back as much of London taxpayers’ money as I can.”

The cannons were retrofitted to operate in the capital’s low emission zone, costing £32,004, and were repainted at a cost of £19,035.

They were also equipped with £3,109.20 of signage and were fitted with £970.50 worth of radios and CD players.

It is understood that the radios were fitted so that officers operating the cannons could listen to news broadcasts during times of unrest, and that they came equipped with CD players as standard.

In the summer of 2014 Stephen Greenhalgh, then Mr Johnson’s deputy mayor for policing and crime, told members of the London Assembly that police and crime commissioners from around the country had advocated the use of ultraviolet dye in the crowd control weapon.