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