Home Secretary Theresa May is said to be in favour of their deployment by police for the first time outside of Northern Ireland amid fears of summer riots.
Both the Metropolitan Police and the Association of Chief Police Officers (Acpo) have requested them to tackle any civil unrest caused by welfare cuts and June’s G8 meeting of world leaders.
The cannons, which would revolutionise riot control tactics and mark a move towards tougher European-style policing, would be available to other forces as a “national asset”, Scotland Yard said yesterday.
Talks are at such an advanced stage that the type of cannon they want to use has even been identified.
The intimidating Ziegler Wasserwerfer 9000 is capable of carrying up to 9,000 litres of water, or even tear gas, which can be fired at rioters at the powerful rate of 1,100 litres per second.
The jet is mounted on a heavy-duty vehicle weighing 29 tonnes that is also equipped with CCTV cameras and searchlights at both the front and rear.
They are understood to cost about £1.3million each, with an order for three being considered.
The Met believes they would be a “valuable option in rare situations” and it has worked with Acpo to develop a formal business case detailing the operational, technical and medical implications of using them.
It is understood that an initial Home Office licence would cover the use of basic water only.
Approvals for the use of tear gas and “smart water”, which contains an invisible marking that identifies a suspect’s presence at a riot, would not be given.
But there are concerns about the direct and indirect injuries often caused by their use in other countries.
Last night, Home Affairs Committee chairman Keith Vaz said Mrs May needed to “tread very carefully” on the issue and called for full public consultation before any decision.
However, the Home Office said no legislation was required and consultation would be “unlikely”.
During the summer riots of 2011, David Cameron said water cannon from Northern Ireland could be shipped over at 24 hours’ notice, but a formal licence for the Met would mean they would have their vehicles ready at any time.
London Mayor Boris Johnson has previously voiced opposition to using water cannons, but the G8 protests represent a serious threat.
Mrs May is concerned about thousands of anarchists and other protesters from North America and Europe converging on Britain next month.
While the actual two-day meeting of G8 leaders is being held at Lough Erne in Northern Ireland on June 17-18, a week of demonstrations is being planned for London beforehand.
The most serious flashpoint is likely to be on Tuesday, June 11 when a “day of action” is planned in central London.
Police fear a repeat of the violent clashes that have marred summits in other countries.
In a call to arms, statements on organisers’ websites say: “London is at the heart of global capitalism.
“The corporations, banks, hedge funds, and billionaires looting our world have names and addresses.
“They are in the glass towers of the City, and behind unmarked doors in Mayfair and Knightsbridge.
“London is the money-laundering den of dictators, the playground of the super-rich.
“The time is now. Let’s come together, and fight.”
A Home Office source told the Sunday Express: “There’s an urgency to get the water cannons by the summer.
“Theresa May is actually pushing for it. The G8 is a worry for them.”
An official Home Office spokesman said: “We will ensure forces have the tools and powers they need to maintain order on our streets.
“We are currently providing advice to Acpo and the Metropolitan Police as they build the case for the use of water cannon.”
Acpo’s spokesman for firearms and conflict management, Chief Constable David Shaw, said: “We are considering how water cannon could be deployed and used.