Anyone watching the news on TV will know the basic scenario. There's a riot of some kind: perhaps political, maybe largely senseless, occasionally related to an international football match. The police duly get stuck in, using horses, or baton charges, or even plastic bullets. And then, just to underline the fact that something really serious is afoot, a number of armoured vehicles will appear. In the manner of giant weeing daleks, they will then fire huge jets of water at their targets, sending them flying across the street, and clearing a space in between the cops and their quarry – or, in a different situation, one set of miscreants and another.

Such are the wonders of water cannon, first used in 1930s Germany, and now part of the armoury of police forces all over the world. Its use in Northern Ireland goes back to 1969, and the police there currently have access to six of the renowned Zieger Wasserwerfer 9000, a few of which were used last summer. On the so-called British mainland, however, the water cannon has usually been thought of as either a signifier for the kind of trouble unique to Ulster, or something best left to the sort of countries where regimes routinely brutalise their citizens (Egypt, Indonesia, China) or the streets have, at some time or other, echoed to the sound of jackboots, and the authorities asking people for their papers (France, Germany, Ukraine).

Until now, that is. In January, it was announced that, in his capacity as mayor of London, Boris Johnson was backing the Metropolitan police's drive to get its hands on water cannon – and was prepared to foot the bill. The Met's chief commissioner, Bernard Hogan-Howe, had briefed Johnson about the water cannon's merits, while other senior Met officers advocated its use. The mayor wrote to Home Secretary Theresa May, serving notice that he was "broadly convinced of the value of having water cannon available" to the Met, and viewed it as the "most economical interim solution that allows the [chief] commissioner to meet his desire to prevent disorder on the streets". Yesterday, Johnson was at it again, telling radio listeners that though water cannon "will be very rarely used", he and the Met are "going ahead with it. There is a consultation and we'll have to see what the home secretary says."

Meanwhile, some senior police figures have been loudly singing the same tune. The Association of Chief Police Officers (Acpo) reckons that, though "there is no intelligence to suggest that there is an increased likelihood of serious disorder", protest against "ongoing and potential future austerity measures" is part of the case for bringing water cannon into the police's range of options. May will be approached any time now, it seems, "in respect of water cannon authorisation"; whether any particular force wants to use it will be up to its chief constable, and its elected police and crime commissioner.

An Acpo report, written by the chief constable of West Mercia, David Shaw (more of whom later), has made the case for its use, and retrospectively identified three occasions when water cannon might have been a sensible tool: 2004's Countryside Alliance demonstration in London; demonstrations outside the Israeli embassy in 2008-09; and the student protests of 2010 – when, as you may recall, irate twentysomethings laid waste to the Tory HQ in Westminster, and some of them managed to menace the Duke and Duchess of Cornwall.

Some of England and Wales's new police and crime commissioners are said to have rejected the idea of using water cannon and have suggested they don't want to share the cost. And plenty of other voices have warned of water cannon's huge dangers. Some have talked about Dietrich Wagner, the 66-year-old Stuttgart resident who was effectively blinded by water cannon in 2010 when German police clamped down on a protest against the building of the city's new railway station (horrifying images of the incident show blood streaming from Wagner's eyes). Such injuries as broken bones and ruptured spleens have been mentioned. Even Acpo's own briefings acknowledge that "water cannon are capable of causing serious injury or even death", and that "there is a range of water cannon footage available online showing injuries caused by water cannon". By way of reassurance, its documents also insist that "deployment criteria as well as the tolerance for disorder or protest vary significantly across the world".

One of the six Wasserwerfer 9000s belonging to the Police Service of Northern Ireland. Photograph: Stephen Barnes/Alamy

Which brings us to perhaps the most powerful argument of all, once elegantly voiced by the home secretary herself. In 2010, May said: "I don't think anybody wants to see water cannon used on the streets of Britain because we have … a different attitude to the culture of policing here in the UK. We police by consent and it depends on that trust between the police and the public."

Interestingly, some of the police officers now pushing for the use of water cannon seem to have only recently changed their minds. In 2011, when he was mulling over that summer's riots, Hogan-Howe said that though water cannon had "been effective" in Northern Ireland, it had distinct limitations, and was "not the answer" to rioting elsewhere in the UK. "In any country, if you haven't used things before then, of course, nobody is going to go willingly towards this new approach," he said. There was also the small matter of money (the trusty Wasserwerfer 9000 costs £1.3m per vehicle). "These things are expensive," he added. "Most of the time, they just sit there doing nothing."

There is some truth to this. In the early 1980s, after such cities as Manchester, Liverpool, London and Leeds had experienced riots, the Home Office borrowed German water cannon to "evaluate" its use, and ended up buying two British-built prototypes. By 1985, they were gathering dust in a Metropolitan police garage in Greenwich, amid what one Guardian report called "growing doubts" that they could ever be used.

Things then went quiet, with water cannon making the news only when it wasthey were brought on to the streets overseas, or British firms sold the requisite kit to foreign governments. In 1997, for example, water cannon made by the defence firm British Tactica were unleashed on demonstrators in the Indonesian capital, Jakarta – and they were used there again as recently as March 2012.

Occasionally, the water cannon has played a role in big British news stories – as in the summer of 2000, when English football hooligans decided to have a go at fans of Germany in the Belgian city of Charleroi, in the usual flurry of lager and broken bar furniture. You can watch the key moment on YouTube: a crowd of troublemakers on a city square being scattered by two formidable jets, squirted from water cannon mounted on the roof of an armoured vehicle bearing the number 10.

England and Germany fans are hit by water cannon in the main square in Charleroi during Euro 2000. Photograph: Action Images

In 2001, there were riots in Bradford, which prompted the dependably level-headed Labour home secretary David Blunkett to suggest that the police might need to have water cannon at their disposal. His contention, it was said at the time, was driven by local people and politicians rather than the police – and once again, any voices clamouring for the introduction of water-based crowd control soon went quiet.

Ten years later, though, came this story's watershed episode: the wave of riots that spread across England in the summer of 2011, after the shooting of Mark Duggan. Towards the end of five days of disorder, David Cameron announced that the police had been given permission to use rubber bullets, and that there were contingency plans for water cannon to be available at 24 hours' notice. May, by contrast, sounded a bit more hesitant. "The police are very clear – they tell me that, at the moment, they don't need water cannon," she said.

In the midst of the riots, Hugh Orde, the president of Acpo, sounded just as noncommittal. "Water cannon are used to deal with fixed crowds to buy distance," he said. "These are fast-moving crowds, where water cannon would not be appropriate … I don't see it as necessary, and nor do the 43 chiefs I spoke to this morning."

In May last year, 4,000 police officers were trained in the use of a Ziegler Wasserwerfer 9000, in preparation for the arrival of that year's G8 summit in Enniskillen, Northern Ireland. It all happened at Longmoor army camp, near Petersfield in Hampshire. With its customary understatement, the Daily Mail flagged up its story as follows: "Enter the water cannon: two years on from the riots that scarred Britain, hundreds of police are training at a secret base with a fearsome new deterrent."

A big question hangs over this story: why is the case for water cannon being made so vociferously right now? "We're as confused as anyone else as to where this has come from," says Sara Ogilvie, a policy officer at the human rights campaign group Liberty. "We're not sure what's changed to make anyone think this would be useful."

By way of one possible explanation, she mentions the fact that Johnson is a "political figurehead with policing responsibilities": the same model that has given rise to the new regional police and crime commissioners, and one she thinks comes with real dangers. Suggesting that the police use water cannon, she says, might have a lot more to with crude politics than the whys and wherefores of law and order.

"Water cannon doesn't sound that dangerous; it doesn't sound that brutal – it sounds like a nice, easy way of letting the police do their jobs, with no one getting hurt," she says. "That's not the case, but it's an easy thing to sell. It seems like a populist policy – and when you've got elected figureheads at the centre of the police, we worry that we're going to see more of it." Water cannon, she insists, are "inflammatory, militaristic and brutal". She goes on: "They're liable to cause panic. They're referred to as 'less than lethal force', which is hardly reassuring. They can really hurt people, and add panic to what might already be a difficult situation."

Police use water cannon to disperse Nationalists rioting in Belfast on 12 July 2001. Photograph: Alan Lewis/AFP/Getty Images

Isn't there also a cultural aspect to this debate, bound up with a deep-seated sense that the water cannon is un-British – unless, of course, we're talking about Northern Ireland?

"Northern Ireland is another story. If there are any lessons we can learn from Northern Ireland, it's maybe that having a big piece of weaponry doesn't guarantee the peace. But I agree: it's not a very British way of policing. It's very paramilitary. And let's be honest: it comes at a time of very low confidence in the police in London. We've had plebgate, we've had the Duggan verdict … I can't think of a worse time for the police to be asking to buy a big military weapon, instead of trying to work out a better way of regaining the trust of the people they're supposed to be there to protect."

Chief Constable David Shaw is on the opposite side of the debate. The author of the Acpo paper that set out the case for bringing water cannon to England and Wales, he says the question of why the proposal has been made in early 2014 is a red herring. The big catalyst, he says, was the 2011 riots. Serious work on public order policing started soon after that, and "it's taken some time to get to where we are," he says. "There's absolutely nothing I could point to and say: 'It's happened now because [of this].' It's just something that's taken time to work its way through."

His briefing paper mentions protest against "ongoing and potential future austerity measures". Does he think we're still in that phase of our recent history? "A year or 15 months ago, there was a possibility that austerity could lead to … some form of demonstration or violent protest," he says. And is that analysis out of date?

"I think it may well be," he says. So does that weaken his argument? "No, no. I don't think it does at all. The whole idea behind this is that it's not a direct response to 2011, or anything we've got now. These things can last 30 years. And things happen over three decades."

Does he agree that introducing water cannon would be a huge change? "Yes I do. I completely get that. And can I just stress: there's a danger that because I've got the term 'national leader' applied to my role here, I'm seen as some sort of zealot for this. My position is that I think it's sort of a necessary thing, but I wish we didn't have to have it. Just like I wish we didn't have to have firearms."

What about the prospect of people being injured, or even killed? "Rather than play things down, and make out that this is just as tool that is completely harmless, and it just gets people wet – what I'd rather do is be completely frank and open about what it can do." He mentions footage of water cannon being used abroad, and the episode in Stuttgart. "I've heard of instances where debris on the ground has been hit by the jet, and that's hit people. I've seen footage of people being knocked down … People can strike their head on the ground." In Northern Ireland, he says, "we have no record of injuries. And the way it would be used, and the training, would be exactly the same."

Still, does he accept that the water cannon is distinctly un-British: one for the French and Germans, perhaps, but not right for here? "I would agree with you, to be honest. But I also think most people would say that it feels un-British, at times, to have to equip your local cop on the street with a ballistic shield, and a Nato helmet, and flame-proof overalls, because people are trying to burn down Croydon." The police, he says, can also use rather un-British baton rounds: "They're a tactic we can use. But just because it's available, it doesn't mean we use it." He accepts that, to some people, water cannon might seem "a little bit alien", but repeats his point: "It's also a bit alien when we see streets on fire."

Exactly how the Home Office will respond to requests to approve the arrival of water cannon is unclear, but a statement issued by a spokesperson gives more than a hint of what is likely to happen: "We are keen to ensure forces have the tools and powers they need to maintain order on our streets. We are currently providing advice to the police on the authorisation process as they build the case for the use of water cannon."

As ever, what they say is made of two very different elements, one of which serves to mask the other. Basically, we are talking about a portent of something associated with the most brutal aspects of state power, wrapped up in the New Labour-ish language of health and safety, and making sure all the right faux-cuddly boxes are ticked.

Whatever happens, the Home Office statement assures, any introduction of water cannon will be subject to a full "community impact assessment", whatever that is. And fair play to Theresa May, Boris Johnson and the Association of Chief Police Officers: they don't have those in Indonesia, do they?

• Chief constable David Shaw, author of the Acpo report on water cannon, will take part in a Guardian web chat on Wednesday 5 February from 1pm.