Sadiq Khan today promised to sell Boris Johnson's controversial water cannon if he is elected London mayor in May.

Johnson bought three second hand water cannon from the German police two years ago, without first receiving authorisation for their use from the home secretary.

Theresa May refused to license the vehicles last year, saying they would put the public at risk and damage the police's reputation. They have since remained unused in a storage facility in Kent.

Revealing his manifesto for London mayor, Khan announced that he would sell the weapons and use the funds to pay for "youth projects aimed at decreasing gang crime"

Khan told Politics.co.uk: "The buying of the water cannon before it was even legal to use was a major failure of Boris Johnson’s which hit London taxpayers in the pocket. Water Cannons are not an alternative to Tory police cuts that have left our communities with insufficient police officers on the streets.

"I’ll sell the water cannon and instead use the money on local schemes which stop young people being sucked into a life of crime in the first place."

Khan's pledge was today condemned by the Conservative candidate Zac Goldsmith. Goldsmith previously described May's refusal to license the weapons as "odd" and has said it was "amazing" that water cannons were not used in the 2011 London riots.

Goldsmith told Politics.co.uk: "The Met Police need to know that the mayor has their back, particularly when they have to make life or death decisions. When they have to make tough operational decisions to keep Londoners safe, I will be right behind them. This means I support the decision to purchase water cannon for the Met as do two thirds of Londoners.

"The next Mayor needs to give the police the tools they need to keep us safe."

Khan's team estimate the refurbished vehicles can be re-sold for £250,000, over £30,000 more than their original purchase price. Even if successful, this would leave City Hall with a loss of up to £100,000 in refurbishment and maintenance costs.

However, it is not clear whether there would be any takers for the vehicles, which are believed to be near the end of their working lives. Home Office inspections of the weapons found 67 significant faults, including accuracy problems and "critical failures" of internal mechanisms. City Hall insists that all serious issues have since been fixed.

There are also a shortage of potential buyers. The Northern Ireland police force already has its own fleet. The Turkish government is one of the few regimes around the world which currently uses them.