A new rift has opened up in the coalition government following the London mayor's controversial decision to force through plans to bring water cannon to mainland Britain.
Johnson decision to purchase the weapons, despite having failed to receive authorisation from the government, has caused a new war of words between the two coalition parties.
Nick Clegg today lashed out at the mayor's "fanciful" support for the weapons.
"My personal view is that I don't think water cannon bought from Germany at great expense are the answer to policing the streets of London," the deputy prime minister said this morning.
"I've not been persuaded by any rational argument that this is the answer," he told LBC.
He said the weapons would damage the "long tradition" of policing by consent in Britain.
"Getting out these big lumbering German water cannon is not going to deter someone from throwing a brick through the window of their local shop," he added.
Clegg was joined by Liberal Democrat Home Office minister Norman Baker, who accused Johnson of trying to "bounce" the government into supporting water cannon.
"This is a regrettable and premature decision by the Mayor of London," Baker told the Evening Standard.
"It's not acceptable to try to bounce the Home Office and gamble with London taxpayers' money to do so. This matter needs careful consideration as to whether this is the kind of policing we want in our country."
Clegg and Baker's comments came as the prime minister came out in support of deploying water cannon.
Asked whether he backed their use in London, his spokesperson said: "The prime minister does. There's an issue of principle here in terms of police having the resources they need."
"It is right that they consider that, but the principle of the police having the resources they need, the prime minister supports that."
Johnson's backing for water cannon has not convinced all in his party however. Earlier this year, several Conservative members of the London Assembly, including Johnson's former policing deputy and his current statutory deputy mayor voted against their introduction.
Johnson himself has also previously been opposed to their introduction. He is believed to have changed his mind after lobbying from the Metropolitan police and his own policing deputy Stephen Greenhalgh.
Asked about his decison at Mayor's Question Time yesterday, he told the London Assembly that "water cannon are not something that I want to see on the streets of London. I don't want them ever to be used."
"In an ideal world they would never be deployed but we have to think about the needs of the police."
The coalition row came as it emerged that the water cannon Johnson ordered, is being phased out by the German authorities because of safety fears.
According to reports, one activist died having been stunned and then run over by one of the machines. An investigation into his death revealed that design flaws had contributed to the accident.
Another activist, Dietrich Wagner was left permanently blinded, after being hit by a jet from one of the machines.
According to officers who had used the machines, hitting a specific target was a "matter of luck" while those controlling the machines had "little idea" of what was going on outside the vehicle.