The British government has refused to let police in England and Wales use second-hand water cannons purchased after 2011 riots.

Home Secretary Theresa May said Wednesday she would not authorize the cannons' deployment in cases of civil unrest. She cited risks including concussion and spinal fractures, and noted a case in Germany in which a man was blinded by a water cannon.

May also said they could be counterproductive in "areas with a history of social unrest or mistrust of the police."

London Mayor Boris Johnson authorized the Metropolitan Police to buy three 25-year-old water cannons from Germany last year.

Johnson said "obviously, I don't necessarily agree" with May's decision.

Water cannons are routinely deployed against rioters in Northern Ireland.