Three unusable water cannon bought by Boris Johnson when he was mayor of London have been sold for scrap, at a net loss of more than £300,000.

Johnson bought the crowd-control vehicles from the German police in 2014, in anticipation of social unrest, without checking whether they could be used on London’s streets. In one of his most humiliating episodes as mayor, the then home secretary, Theresa May, banned them from use anywhere in England and Wales. It left the capital’s taxpayers with three expensive white elephants.

The current mayor, Sadiq Khan, pledged to claw back as much money as possible on the redundant vehicles by selling them. But after almost two years the mayor’s office admitted defeat in its attempt to find a reputable buyer.

It announced on Monday that it had agreed to sell the vehicles for just £11,025 to Reclamations Ollerton, a scrap metal yard in Newark, Nottinghamshire.

The fee recoups 3.4% of the £322,834.71 spent on the vehicles since 2014.

The 25-year-old vehicles cost £85,022 in 2014, but they were found to be riddled with faults and required expensive modification to make them roadworthy. This included £32,000 to comply with the city’s low emission zone, and almost £1,000 on new stereos.

The former London mayor Boris Johnson bought the crowd-control vehicles without checking whether they could be used on the capital’s streets. Photograph: Victoria Jones/PA

Officials had hoped to sell the vehicles for up to £43,000, but despite the improvement work, no buyer could be found.

Under conditions of the original sale by the German federal police, the vehicles could only be sold on to a European policing or civil protection organisation, as a guard against them being used by a repressive regime.

The Ministry of Defence has been trying to help offload the vehicles since December 2016 but no European authority wanted them. The mayor’s office said: “The length of time they took to sell was due to lack in interest from potential buyers, including the UK government and overseas authorities.”

The failure represents a setback for Khan, who had promised to sell the vehicles in his election manifesto and spend the money on youth services. The money he has recouped does not even cover the estimated £12,000 bill for insuring the vehicles [pdf] since he was elected in May 2016.

But he still claimed that by selling the vehicles for £3,675 each for scrap he had met his election pledge.

Khan said: “For too long, London taxpayers have had to bear the brunt of Boris Johnson’s appalling botched water cannon deal. This has been another waste of taxpayers’ money by Boris Johnson. Londoners continue to live with his vanity.

“I am pleased we have managed to finally get rid of them and I made an election promise to Londoners that I would claw back as much of this cash as possible, and pump it into helping young people at risk of being affected by crime and giving them better life opportunities.”

The Green party joint leader and London assembly member Siân Berry said more money would have been saved if the vehicles had been scrapped sooner.

She said: “This is the final proof that Boris’s water cannon stunt was just money down the drain. It’s an affront to Londoners that any public money was spent on what is essentially a weapon against the people. It’s a shame more of the money couldn’t be recouped and spent on young Londoners, but I’m glad that no one will be using the water cannons against any people as it was rightly deemed inhumane.”

Steve Reed, the shadow minister for civil society, said Johnson should be held to account for wasting so much money during a squeeze on public services.

Boris Johnson’s unused and unusable water cannon wash £300k of public money down the drain - he should be held to account for this at a time when London’s schools and police are facing such severe cuts https://t.co/PrIkRm8cuX — Steve Reed (@SteveReedMP) November 19, 2018

It was understood that before buying the cannon, Johnson had been given assurances by the then prime minister, David Cameron, that they would licensed if there was another riot on the scale of the disturbances experienced in the summer of 2011.

The Home Office had asked for a number of safety assurances about the cannon. These were met by the mayor’s office for crime and policing and Scotland Yard before May intervened to outlaw them outright.

Sources close to Johnson said: “Boris made tackling crime his number one priority when he came into office in 2008.

“During his time as mayor he reduced the murder rate by 50% and brought overall crime down by 20%. This is a legacy the new mayor has sadly been unable to match.”