Stringer/Taiwan / Reuters

Police commissioners have urged the Met to put ultraviolet dye in its new water cannon in order to track people hit by the weapons, Boris Johnson's policing boss has said.

Stephen Greenhalgh, Deputy Mayor for Policing and Crime, told members of the London Assembly that police and crime commissioners from around the country had advocated the use of "SmartWater" in the crowd control weapon.

The substance would remain invisible until those hit by the water had an ultra-violet light shone on them, at which point it would glow fluorescent yellow and identify them as having been at a protest or other public order situation.

SmartWater is normally used to invisibly mark valuable belongings such as laptops so that the police can trace their owners if they are ever stolen, and also to permanently stain the clothes of people involved in burglaries.

Greenhalgh said there was a "range of views" among police commissioners about the plans to purchase the weapons.

"[Some PCCs] recognise that there is a case for their utility, although they would not want to see them being used," he explained.

"I’ve equally had other PCCs who are vehemently in favour of it and suggesting putting SmartWater into the water cannons so they can identify people. There’s a range of views."

Greenhalgh made the revelation at the London Assembly's Police and Crime Committee, which scrutinises the Mayor's record on policing.

Police commissioners from other forces have no direct decision making power over the Metropolitan Police, but their advice is often sought on matters that could have national implications.

The Mayor of London has bought three water cannon for the Metropolitan Police, though the Home Secretary has yet to authorise their use.

The Metropolitan Police says the cannon will fill "a gap in public order tactics" but there are concerns that the weapons, which can kill and blind people, could constitute a disproportionate use of force by the police.