Controversial water cannon intended for use on the streets of the British mainland will be sent to London next week.
The Metropolitan police have opted to buy three second-hand machines from the German police.
A spokesperson for the German federal police said the sale had been completed and the machines would be sent to London next week.
The London mayor, Boris Johnson, has ordered the water cannon before the home secretary, Theresa May, decides whether or not to authorise their use on the streets of the British mainland.
A spokesperson for Germany's federal police said: "Contract negotiations have been completed, and the water cannons will be dispatched to London next week."
They are expected to be transported by road and sea. Once they arrive it is expected to take several weeks for them to be made ready for use if public order chiefs decide they are needed.
Johnson's administration decided to buy the second-hand machines for £218,000 for possible use this summer to tackle potential disorder. Critics say the case for their purchase has not been made and so far the Met is the only force to support their use; other major forces are against deploying water cannon.
The machines have been used frequently in Northern Ireland but never on the British mainland.
A spokesperson for the mayor's office for policing and crime said: "These are an operational police asset and their movement is subject to security considerations. Before they are ready to be deployed a number of modifications need to be made. These include the fitting of CCTV and adaptations to bring them into line with the standards used in Northern Ireland. This is expected to take a number of weeks."
In a statement, the Met promised the cannon would not be used unless the home secretary authorised their use.
"The MPS have now purchased three water cannon from Germany following a decision by the deputy mayor for policing and crime," it said. "They will be transported back to the UK and will then undergo renovation. They were purchased at this time as it was cost-effective. However, we stress that these will not be deployed until or unless the home secretary authorises the use of water cannon in England and Wales."
The completion of the sale comes in the same week that two German police officers went on trial over the use of Wasserwerfer 9000 water cannon during a demonstration. A protester, pensioner Dietrich Wagner, lost his sight in one eye after being hit by a jet of water in Stuttgart on 30 September 2011.
Wagner's lawyer, Frank Ulrich Mann, claimed police across Germany were less keen on the use of water cannon since the public backlash after the Stuttgart protests. They were now used mainly to burst windows ahead of raids on high-rise buildings.
He said: "I would advise the London mayor against the purchase of water cannons. Especially if their use isn't trained and practised regularly. They are dangerous tools that can lead to grave injuries".
The use of water cannons were first issued and used during the troubles in Northern Ireland and caused injury and controversy. It would now seem that the Government are expecting more trouble on the Mainland due to protests about austerity and uncontrollable immigration.