London mayor Boris Johnson has decided to buy three water cannon for the Metropolitan police.

The purchase from the German federal police is the first by a force in Great Britain. Critics say the decision smacks of "arrogance". It comes before Theresa May, the home secretary, has reached a decision on whether to sanction their use.

A document on the mayor's policing and crime website states: "Following the mayoral consultation, approval is sought to purchase three water cannon with immediate effect.

"The timing of this decision is driven by the enhanced risk that the water cannon, currently available to be purchased secondhand, are sold to another European police authority. The saving represented by this purchase is so significant as to justify the risk caused by the delay in the licensing process."

Approval was given for the purchase by the deputy mayor for policing. The cost will be £218,000 and the mayor justifies the speed of purchase by saying the water cannon are needed in case of disorder this summer in London.

The document says: "Purchasing at this time considerably improves the possibility that the Metropolitan police would have the tool to deploy in the summer, when, although there are no expectations of violence, such tools are most likely to be needed.

"The water cannon will not be deployed until (and unless) the home secretary authorises their use in England and Wales."

Jenny Jones, of the Green party, condemned the buying of water cannon for use on London's streets: "Pre-empting the home secretary's decision is rash and smacks of arrogance on the mayor's part, not to mention a possible waste of taxpayers' money at a time when the Met is making huge cuts.

"It's also ignoring the views of the vast majority of Londoners who don't want water cannon. If the mayor had actually taken the time to look his own consultation he would have seen 98% of people opposed the plans for water cannon."

Joanne McCartney, Labour London Assembly member and police and crime spokesperson, said: "To rush this through is typical of Boris's slapdash approach to issues of crucial importance to Londoners. It is telling that 20 out of 25 Assembly members – from all parties – voted against their purchase earlier this year."

The Met commissioner, Sir Bernard Hogan-Howe, has pledged that water cannon would be "rarely used and rarely seen".

It has been used regularly in Northern Ireland but never in England.

A spokesman for the mayor's office for policing and crime said: "This decision follows the mayor's announcement in March that he would support the commissioner's request to purchase three water cannon from the German federal police at a considerably reduced rate, and much more expeditiously, compared to commissioning new devices.

"In order to secure them the deputy mayor for policing and crime has authorised the Met to proceed with procurement."

The vehicles need modifications in order to make them suitable for use in London. The spokesman said: "By purchasing them now we are able to save over £2.3m compared to buying new devices, and ensure that they are ready as quickly as possible, subject to the home secretary authorising their use."

The Home Office denied Johnson had either pre-empted Theresa May's decision or been given a clear signal she would authorise the use of water cannon. A spokesperson said: "We are keen to ensure that the police have the tools and powers they need to maintain order on our streets.

"Chief Constable David Shaw, as the national policing lead, has written formally to the home secretary to request that water cannon be authorised for use by the police in England and Wales.

"The home secretary is considering his request."

In a statement the Met police praised the decision: "We welcome the deputy mayor for policing and crime's decision to purchase three water cannon from Germany. We stress that these will not be deployed until or unless the home secretary authorises the use of water cannon in England and Wales.

Following learning from the riots of 2011 the MPS requested that water cannon be made available to address a gap in public order tactics, allowing us to more effectively and safely tackle the most extreme disorder.

"We believe that as a tactic they would be rarely seen and rarely used on our streets. This is a point we have tried to make clear throughout the consultation process conducted by the mayor's office. The timing means we able to buy them at a fraction of the cost of newly-commissioned water cannon."