Boris Johnson will "live to regret" bringing water cannon to London's streets, a close aide to the mayor has told Politics.co.uk.
The senior member of Johnson's team suggested that the mayor did not understand the power of the weapons, which have caused the blinding and even death of protesters in Germany.
"I don't think there is an understanding of what a powerful weapon [water cannon] are," they said.
"It's like being hit with a baseball bat. I think he will live to regret it."
The comments from a normally loyal ally of Johnson are a sign of growing disquiet at City Hall over his decision.
Several members of Johnson's team have previously warned against buying the three secondhand water cannon from Germany.
Four Conservative London Assembly members, including the mayor's former policing deputy Kit Malthouse, voted against their purchase earlier this year.
Johnson sparked a row with the Home Office earlier this month after pushing ahead with the purchase of the weapons, despite failing to gain any authorisation for their use from Theresa May.
German authorities are currently in the process of phasing out the Wasserwerfer 9000 model, purchased by Johnson, due to safety concerns.
The Home Office has yet to carry out any safety checks on them.
Earlier this year the London Assembly heard from German pensioner Dietrich Wagner, who was blinded by the weapons during a protest in Stuttgart.
Wagner warned that it would be a "big mistake" for Boris to bring them to the UK.
"The police surrounded us and used water cannon to threaten the demonstrators," he wrote of his experience in the Telegraph.
"Ignoring their own rules and regulations for use, they blasted the water jet directly into my face, at a range of only 15 metres, and gradually increased the pressure.
"This smashed through my cheekbone and if the spray had continued for just one more second it would have hit my brain and I would have lost my life."
The weapons bought by Johnson have a notoriously poor safety record in Germany.
Leftwing activist Günter Sare died in 1985 after being stunned and then run over by one of the weapons.
Officers who have used them, say it is a "matter of luck" whether they hit their intended target and have complained that poor design in the cockpit makes it incredibly difficult to know what is happening outside.
Speaking at the State of London debate last night, Johnson defended his purchase of the weapons.
"I don't want these things to ever be used on the streets of London," he insisted.
"However I have to respect the advice of the police and their strong view that there might be circumstances when they could save life and limb and damage to property if they had this tool."