A London Assembly report has found that the Metropolitan Police has failed to make a "convincing case" for buying three water cannon.
The police and crime committee says that, with no specific intelligence of disorder, it is unclear why there is a "rush" to get them.
London Mayor Boris Johnson is consulting on plans to pay up to £100,000 each for three second-hand water cannon from Germany.
The Met has said the water cannon would be "rarely seen and rarely used" and it claims to have public support for the idea.
The vehicles have been used in Northern Ireland but they are not currently authorised in mainland Britain.
The home secretary must approve their introduction in England and Wales.
Mayor Boris Johnson's controversial plans to allow the Metropolitan Police to purchase water cannon will come under scrutiny at City Hall for a second day.
Assistant Commissioner Mark Rowley and Deputy Mayor for Policing and Crime Stephen Greenhalgh will appear before the Assembly today to answer questions on the Mayor's proposals.
Chair of the Police and Crime Committee Joanne McCartney AM said: "Met Commissioner Sir Bernard Hogan-Howe has said water cannon in London would be 'rarely used and rarely seen'. We have yet to hear a convincing case for when and how they would improve safety for all on London's streets."