Water cannon have arrived in mainland Britain and could be used against protesters within months, Boris Johnson confirmed today.

The controversial weapons arrived on British soil in the past few days and are now being adapted for use on London's streets.

"They are being prepared for use in this country and we await the certificate [authorising their use] from the Home Office," Johnson told the London Assembly.

Johnson said that he did not want to see the water cannon used, but could see circumstances where they may be useful to control crowds.

"If there arise circumstances where life and limb is in jeopardy and where property could be protected... then yes of course it might be conceivable that water cannon could be useful," he said.

Metropolitan Police commissioner Hogan-Howe told assembly members the cannon were now being adapted for use in London.

He said surveillance cameras would be fitted to the weapons to monitor their use.

The Met are also considering allowing the weapons to fire identifying "smart water" at protesters.

Hogan-Howe said the adaption and training process would take "a few months", but insisted they could be used sooner if necessary.

"Needs must, if something awful happened [in London] then we'd all have to think again, but broadly I think this is going to take a few months to get to a stage where we can deploy them."

He dismissed suggestions from former Northern Ireland police commissioner Sir Hugh Orde, that the water cannon should be routinely deployed at protests.

"Our ambition would be for a generation that they would never be seen."

Asked what would happen if the Home Office refuse to authorise the weapons, Johnson insisted he would easily be able to sell them on to another country.

However, he said he could not "imagine the circumstances" whereby he would sell them onto a brutal or repressive regime.

Johnson was at pains today to suggest that the weapons would rarely be used by police.

He is under growing pressure at City Hall to row back from his enthusiastic support for the weapons.

Last month, a senior aide to Johnson told Politics.co.uk that the mayor would "live to regret" his decision to deploy water cannon in London.

"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 water cannon bought by Johnson are currently being phased out in Germany after one protester was blinded by the weapons and another killed.

Those who have used the weapons, say it is a "matter of luck" whether they hit their intended target and have complained about a lack of visibility in the vehicles.