Gerard Batten, whose party has veered towards the far right under his leadership, falsely suggested the Metropolitan Police could deploy the equipment - which the force does not own - to quell disorder as demonstrations take place on Friday.
Police are braced for potential unrest as several pro-Brexit groups, including Ukip, hold rallies in the capital on the day the UK was originally scheduled to leave the EU.
“There are reports that water cannon may be deployed tomorrow in London, & that some of the police may try to aggregate [sic] and provoke Brexiteers,” tweeted Mr Batten, giving no examples of the “reports” he cited.
“There is always the danger of provocateurs planted to cause trouble,” he added. “I hope it is not true but I call on everyone present to be peaceful.”
Responding on Twitter, the Metropolitan Police said any suggestion it could use a water cannon was "indisputably false".
"No water cannons are owned or used by the Met Police," it added. "Any suggestion that we are intending to use water cannons or seeking to agitate any protestors is baseless and false."
A Scotland Yard spokesman told The Independent Mr Batten's claim was “nonsense”.
The use of water cannons to disperse disorder is banned in Britain.
Mr Johnson paid £322,000 to acquire and refit the second-hand machines, which were not licensed for use in the UK. The next year Theresa May, then home secretary, banned their use in riot situations.
The cannons were sold to a Nottinghamshire scrap dealer for £11,025 in November 2018, a loss of almost £311,000.
Scotland Yard said there was no indication protests planned for Friday would not be largely peaceful, but added it was ready to respond "if disorder breaks out".
Ukip, Tommy Robinson, the UK “yellow vests”, the Democratic Football Lads Alliance, and Leave Means Leave campaigners are all holding demonstrations on the day MPs vote on Theresa May's Brexit deal in parliament.
Counter-protesters are planning to meet them after accusing anti-Islam groups of using Brexit as a “platform to spread their extreme far-right agenda”.
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