Boris Johnson's water cannon farce reveals why he will never be prime minister

Future historians will look back on yesterday as the day Boris Johnson's prime ministerial ambitions finally washed away into the muddy waters of the Thames.

His full-body soaking at the hands of the home secretary in the House of Commons was so brutal it was almost difficult to watch. Theresa May's demolition of Johnson's case for bringing water cannon to the streets of London was so comprehensive, so cutting and so utterly devastating for the London mayor, you half expected the Metropolitan police to apprehend her for common assault.

With an increasingly meek looking Johnson sat behind her, May explained that the 25-year-old second hand German water cannon Johnson bought without her permission, would "pose a series of direct and indirect medical risks," to protesters, including "musculoskeletal injuries such as spinal fracture, as well as other serious injuries such as concussion, eye injury and blunt trauma". One 66-year old German protester had been completely blinded by similar water cannon in Stuttgart, she added.

With Johnson shrinking into his seat, the home secretary explained that she also objected to the weapon's use in principle, explaining that: "I am acutely conscious of the potential impact of water cannon on public perceptions of police legitimacy.

"As a number of chief constables argued, in areas with a history of social unrest or mistrust of the police, the deployment of water cannon has the potential to be entirely counterproductive. This country has a proud history of policing by consent, and this decision goes to its very heart," she added, devastatingly.

The assault on Boris's credibility didn't end there. As the London mayor looked on, May explained that independent checks on the weapons had found no fewer than 67 significant faults with the vehicles. An assessment by the Centre for Applied Science and Technology discovered a whole series of major and potentially fatal problems with the weapons bought at a knock-down price by the mayor.

Many of the worst safety and operations faults have been redacted from the final report. However, those revealed by the Home Office yesterday include:

1) Severe accuracy problems

Inspectors found the vehicles lacked proper external cameras necessary for operators to accurately aim the water jets. The cameras the vehicles did have were faulty and prone to serious condensation problems leaving operators unable to see what or who they were hitting through the "fog". The lack of proper external lighting only makes this situation worse, they added. "It is difficult to see when a water jet contacts an object," the report explains. Operators of the vehicles in Germany have also reported that it is more "down to luck than judgement" whether the weapons are able to hit the correct targets.

2) Faulty communication systems

The noise of the water pumps in the ageing weapons was found to be so loud that it was almost impossible for operators of the vehicles to properly communicate with each other. In fact, noise levels were so high the inspectors suggested ear protection may be necessary, further complicating communication within the cab. Amusingly, given the repeated complaints about sweltering conditions in Boris's 'new Routemaster buses,' the inspectors also found heat levels within the vehicles were well in excess of acceptable working conditions. The idea of sweltering operators, staring into a foggy camera, while being half deafened by the roar of the pump, does not sound like the safest environment in which to work, especially when operators have a potentially lethal weapon in their hands.

3) Recurrent faults

The inspectors found the weapons have as little as two years' operational life left before they have to be turned into scrap. Recurrent problems with the pump and the "critical failure" of the tank would be hugely costly to repair and there could be significant problems finding spare parts for vehicles that are now at the end of their operational life. Day-to-day operation and maintenance of the weapons would also be complicated by the lack of any manuals available in English. Meanwhile the time needed to make essential repairs could leave the weapons out of service for significant periods of time.

In short, the Home Office found the weapons to be seriously dangerous, hugely difficult and expensive to maintain and operate, and shortly in need of a trip to the scrap yard.

Given the majority of these problems were public knowledge in Germany before Johnson agreed to buy the weapons, it is incredible that he still went ahead anyway. It is especially baffling given that he was repeatedly warned against buying the weapons, not just by his opponents, but by his own former policing deputy and his own party on the London Assembly. Last year, when Johnson pushed ahead with their purchase anyway, a senior member of his City Hall team told me that Boris would "live to regret" his decision. How right they were.

In fact, Johnson's more sensible advisers aside, the only people to have come out of this sorry saga well are the negotiators of the German police force. The ingenuity and financial sense of the Germans has been somewhat called into question in recent weeks. However, they can hardly be faulted for their prudent management of this particular deal. It is difficult at the best of times to dispose of obsolete and life-threatening military hardware. But for the Germans to get a foreign politician to actually pay 300,000 euros for the privilege of disposing of the defunct weapons for them was a particularly impressive feat.

As others have commented, Boris has not had a very good start to his time back in parliament. Within weeks of returning to the Commons he has been publicly humiliated by not one, but two senior figures in his party, while his contributions to the Chamber have been largely forgettable.

However, things could have been so different. There was a brief window before the election when the polls predicted that Cameron would soon be out of Downing Street. At that point Johnson was actively preparing to ascend to high office. Had the polls been right, a Conservative defeat in May would have almost certainly forced the party to look for a more appealing leader with a record of winning elections. Boris, with his voter-friendly persona would have fit the bill and he would be right now preparing to lead his party back into government. Sadly for him, the polls were wrong and he has instead been reduced to acting as the comic relief, while rivals like May get on with the serious business of national government.

And it is in this respect that Theresa May's performance yesterday was so impressive. By so thoroughly explaining the case against water cannon, even in the face of criticism from her backbenches, May appeared both principled, ruthless and a credible leader of her party. Boris, by contrast looked faintly ridiculous. If the London mayor had not learnt from the experience of his friend Michael Gove about what happens when you risk taking on the home secretary, he certainly had by the time he left the Commons yesterday.

And it is precisely Boris's capability as a political leader which will now be seriously called into question.

With less than a year to go until Boris leaves City Hall, eyes are already turning to his record as mayor. But after seven years in charge of London it is difficult to discern any substantial achievement under his name. With the arguable exception of his cycling initiatives, Johnson's contribution to London has been high on rhetoric and supremely low on substance. Like a distracted school boy, Johnson has spent his time endlessly seeking new pieces of kit to play with. From his rental bikes, to his bespoke Routemaster bus, with faulty air conditioning and an open rear platform that never opens, to his cable car that almost nobody uses, to his bonkers plan for an island airport, Boris has become increasingly obsessed with finding the next mayoral toy to play with. The now defunct water cannon was the latest in a long line of vanity playthings for London's increasingly dilettante mayor.

Meanwhile the big serious issues facing London, like its growing housing crisis and widening inequality, have gone largely untouched by the mayor. Given the choice between a dull but worthy problem and a fun but frivolous hobby, Boris has always opted for the latter.

Of course this is something you can largely get away with in a job like London mayor, which many Londoners see largely as a ceremonial one. But it is not an approach that is survivable in the cut and thrust of national politics.

In recent weeks both May and George Osborne have shown themselves to be serious and skilful politicians, capable of outsmarting both their external opponents and their internal rivals. When Cameron does stand down, it is easy to see one of these two figures stepping up to replace him.

After his demolition in the Commons yesterday, it is increasingly difficult to ever see Boris doing the same.