The second-hand cannon have already been purchased from Germany at a cost of almost a quarter of a million pounds to the taxpayer, with Met Police riot squads already training with the contraptions in Kent.
But in a crippling blow to Johnson, seen as a key rival of May's for the future Conservative leadership, the Home Office announced on Wednesday that it would not take any decision to licence the water cannon before the General Election on May 7.
(L-R) Home Secretary Theresa May, Metropolitan Police Commissioner Sir Bernard Hogan-Howe and Mayor of London Boris Johnson
Green party London Assembly member Baroness Jenny Jones confirmed the decision to the London Assembly on Wednesday.
“The Mayor’s decision to buy these weapons in advance of the Home Secretary’s approval was rash and arrogant, and a waste of public money," she said. "I’m pleased the Home Secretary was able to see what the Mayor could not. Water cannon is an indiscriminate military weapon that has no place on our streets.”
The move to use the machines sparked protests last year backed by German pensioner Dietrich Wagner, who was left with horrific injuries and blinded after being hit by a water cannon during a protest in Stuttgart in 2010.
Wagner was left unconscious, his eyes irreparably damaged, when he took the full force of a water cannon to the face during an environmental demonstration.
He warned: "Ever since I was hit my life has drastically changed. I can’t drive, go shopping, read or do any of the things I used to do. My message is police need to be aware that they are not just a big shower, they are lethal weapons and do serious bodily harm."
WARNING: GRAPHIC IMAGE BELOW
Wagner, after he was injured by the water cannon
"The Home Secretary’s refusal to approve water cannon for use on London’s streets should be a clear signal to Boris Johnson that he is the only one who thinks this ill-judged proposal is a good idea," Labour London Assembly spokeswoman Joanne McCarthy said.
"Water cannon are not only extremely dangerous, they are bluntly indiscriminate tools which should have no place in our capital city. It’s time for the Mayor to accept he was wrong and to sell the water cannon he has already bought so we can reinvest the money in things the Metropolitan Police actually need. If he refuses, the public will undoubtedly conclude that he is more interested in continuing his militant posturing than doing what is best for the capital.
"The London Assembly have spoken with a clear and cross-party voice on this subject – we do not want water cannon in London. I am glad the Home Secretary agrees with us."
The Liberal Democrats have also previously condemned the water cannon purchase as "reckless", insisting there is not enough evidence that the tactic is effective in maintaining order.
Water cannon have never been used on the British mainland, although they have been used in Northern Ireland.
A spokesman for the Mayor's office insisted public opinion backed the use of water cannons. "Independent polling shows that the majority of Londoners support the police having water cannon at their disposal for use in exceptional circumstances. The Mayor has made his position clear and awaits the Home Secretary’s licensing decision."
The Metropolitan Police has repeatedly stressed the benefits of the cannons as a crowd control measure, in the aftermath of both the 2011 London Riots and the student fees protests. Johnson has previously said he would not have backed the use of cannons against the students during the London protests.
But the opposition says most Londoners are unaware of the full picture. More than 35,000 people have signed a petition against the use of the machines.
The London Assembly voted against the purchase of water cannon, with all Labour, Liberal Democrat and Green AMs united against the plan.
Senior Tories also opposed the measure, including former deputy mayor for policing Kit Malthouse and deputy mayor Victoria Borwick.