Theresa May says she has decided not to authorise the use of water cannon by police in England and Wales.
The Home Secretary told MPs she was ruling out allowing forces to use the Ziegler Wasserwerfer 9000 after a thorough review.
Scotland Yard bought three of the water cannon second hand from German police last year at a cost of more than £200,000.
In a statement, Metropolitan Police say while they respect Theresa May’s decision, they are “naturally disappointed” they will not be able to use the water cannons.
“We believed allowing police the option of deploying water cannon, even though they would be seldom seen and rarely, if ever, used, was a sensible precaution,” the police statement said.
The decision is a blow for London Mayor Boris Johnson, who endorsed the purchase of the cannon even though they cannot be deployed without authorisation from the Home Office.
But the London Mayor told Sky News the machines were “perfectly serviceable” and he intended to keep them “in reserve”.
Mrs May said an assessment had highlighted the risk of water cannon causing serious injuries such as spinal fractures, eye injuries and blunt trauma.
She also said she was “unconvinced” that the 25-year-old machines bought by the Met Police were fit for use – with 67 issues found even after they had undergone repairs.
She told MPs senior police had said water cannon had “limitations” when it came to “fast, agile disorder” and could even attract a crowd to a vulnerable location.
And she said deployment of the weapon could have a negative impact on public perceptions of the police and the British principle of policing by consent.
Mrs May added: “This country has a proud history of policing by consent and this is a decision which goes to its very heart.
“But where the medical and scientific evidence suggests that those powers could cause serious harm, where the operational case is not clear, and where the historic principle of policing by consent could be placed at risk, I will not give my agreement.
“The application for the authorisation of the Wasserwerfer 9000 water cannon does not meet that high threshold.”
Following the announcement, Mr Johnson said he does not “necessarily agree” with Mrs May’s decision.
The London Mayor, also MP for Uxbridge and South Ruislip, told MPs the purchase of the water cannon had been backed by police, the Prime Minister and the people of London.
Speaking to Sky News, he defended the £218,000 outlay and said police would continue to train with the German weapons in case a situation arose when they were needed in the future.
Mr Johnson has previously volunteered to be sprayed by water cannon to demonstrate they are safe.
The machines have never been used on the British mainland, although they have been deployed in Northern Ireland and were used during disturbances in Belfast earlier this week.
The Police Service of Northern Ireland (PSNI) said water cannons helped protect officers during times of serious disorder.
Alistair Finlay, former assistant chief constable of the PSNI, wrote to the Home Secretary on behalf of the chief constable.
“In our experience water cannons enhance the tactical options available to police and are best utilised in support of, and supported by, other tactical options, such as protected officers and attenuating energy projectiles,” he said.
“It is clear that the use of the water cannon can be effective in keeping those involved in serious disorder at a distance from the police lines.”
A Met Police spokesperson said officers would continue to be trained to use the water cannons in the event they are deployed to help their Northern Ireland colleagues.
However the type of water cannon used by the PSNI are different from the ones purchased by police in London.