The home secretary's decision to refuse the use of water cannon in England and Wales is "disappointing", the Metropolitan Police has said. 

London Mayor Boris Johnson could be on the lookout for a buyer after he purchased three second-hand cannon from German police last year at the taxpayers' cost of £218,000.

Officers from the Met have undergone training on the vehicles and considered water cannon to be a more safe and effective way to deal with serious public disorder events.   

Theresa May told the House of Commons the decision on whether to authorise the use was a "serious one". 

She said: "Water cannon, without safeguards, have the capacity to cause harm. 

"Since I became home secretary, I have been determined to give the police the powers and tools they need to cut crime and tackle disorder on our streets.

"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."

She also had concerns about the operability of the cannons, which are 25 years old and have 67 outstanding faults that would have needed to be repaired before being deployed.

In a statement, the Met, said: "We fully respect the home secretary's decision but we are naturally disappointed. 

"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 which would allow us to deal with a number of specific public disorder situations more safely and effectively than we are currently able to.

"We understand that any changes to the way police are able to use force in any scenario must be very carefully considered and we know the home secretary has applied detailed scrutiny to the evidence before coming to her decision.

"We will be retaining our devices to ensure our officers are properly trained for this eventuality.

"This will also give us access to equipment that is already used elsewhere in the UK, but only on specific political approval, as is appropriate, and one that is likely to be granted only in the most extreme of circumstances."

Water cannon are used in Northern Ireland and were deployed as recently as Monday during the annual Twelfth of July celebrations.  

According to the BBC, the total cost of purchasing the three vehicles, including transportation, re-fits, maintenance and officer training, is £328,883.

Shadow home secretary Yvette Cooper said Mrs May's decision was "exactly right".