water cannon
Police are able to use water cannons against rioters in Northern Ireland, but not in England and Wales (Getty)

The Metropolitan Police have said they are "naturally disappointed" by the Home Secretary's decision to reject applications to use water cannons in England and Wales in case of disorder.

Theresa May confirmed in the House of Commons she has turned down Mayor of London Boris Johnson's request to use the cannons as, without safeguards, they have the "capacity to cause harm".

She added: "While evidence suggests that these water cannons are unlikely to result in serious or life-threatening injuries as currently built and used as envisaged, the assessment nonetheless poses a series of direct and indirect medical risks from their use.

"These include the possibility to cause primary, secondary and tertiary injuries, including musculoskeletal injuries such as spinal fracture, as well as other serious injuries such as concussion, eye injury and blunt trauma."

May referenced 66-year-old German protester Dietrich Wagner, who was blinded by a water cannon in Stuttgart in 2010, as evidence of this.

Johnson applied for permission to use water cannons during instances of public disorder in the wake of the 2011 summer riots, which began in London and escalated to other parts of England.

Following the announcement from May, the Met said they are "disappointed" but fully respect the decision.

A spokesperson added: "We believed allowing police the option of deploying water cannons, even though they would be seldom seen and rarely, if ever, used, was a sensible precaution that would allow us to deal with a number of specific public disorder situations more safely and effectively than we are currently able to.

"We presented our evidence to the mayor. He supported our request, and funded the purchase of three vehicles. Our officers have been able to train with these for several months. This has been of significant value in developing our understanding of how to deploy the vehicles and what tactics we should adopt in the very rare event of extreme disorder.

"However, 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."

Johnson, who purchased three second-hand Ziegler Wasserwerfer 9000 water cannons in the wake of the 2011 London riots for £218,000 ($419,000), told MPs he does not "necessarily agree with the conclusion".

Johnson, who offered to be shot at with a water cannon in 2014 in order to prove their safety, added: "But can I remind her that the decision to buy the Wasserwerfer was taken in light of the strong support of the Commissioner of the Metropolis for this operational tool of crowd control, of the strong support of the prime minister and indeed the strong support of the people of London, as expressed in a poll that found 68% were in favour, and indeed in the interests of economy since we were able to buy these machines and thereby save £2.3m.

"No one on either side of the house wants to see the deployment of water cannon anywhere in the United Kingdom and I fail to see the physiological difference between the people of England and Wales and the people of Northern Ireland."