There are circumstances under which the police might need to use reasonable force to subdue political demonstrations that turn violent. But what is so damning about the video footage that has emerged from last week's G20 protests in London is that there is nothing in the slightest bit reasonable about the force that was used against Ian Tomlinson.

Mr Tomlinson was plainly walking away from riot officers when he was struck. It is impossible to say for certain that this assault by a baton-wielding riot officer brought on Mr Tomlinson's heart attack only a few minutes later. But most people will find it impossible to disconnect the two events.

It would, of course, be foolish to second guess every decision made by police officers in the heat of such an engagement. But that cannot mean that anything goes. This incident looks like a serious breakdown in discipline. It also serves to underline broader concerns voiced about the police tactics at last week's demonstration. Officers corralled protesters into separate enclosed areas and then forcibly dispersed them. It was in just such a "kettling" operation that Mr Tomlinson was assaulted.

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Almost as disturbing as the assault itself was the misleading response of the police when they were first probed on the incident. They made no mention of contact between Mr Tomlinson and their officers before he collapsed and briefed that other protesters had impeded police medics in their efforts to help him. It was only when this new footage emerged that the police admitted they might have a case to answer.

As for their attempts to present their involvement as merely shielding Mr Tomlinson from an angry mob, this was reminiscent of the false information circulated in the wake of the mistaken shooting of Jean Charles de Menezes in London in 2005. The public were told on that occasion that Mr Menezes' behaviour and clothing had given them cause for suspicion. These lies were exposed by CCTV footage from Stockwell Underground station, just as the police's account this week has crumbled in the light of these latest images.

The Independent Police Complaints Commission has launched an inquiry into the apparent assault on Mr Tomlinson and will decide whether charges should be brought against officers. But this terrible case reflects deeper problems in the police, the most glaring of which is an entrenched culture of unaccountability.

No police officer has been convicted of a firearms offence in 15 years, despite the deaths of 30 individuals, many of whom were unarmed. Rather than learning from this poor record, the police have relaxed their guidelines for firearms officers. New rules of engagement were introduced several years ago (without a sniff of public consultation or parliamentary debate) allowing the police to shoot suspected suicide bombers without issuing a warning or identifying themselves. This unaccountability is accompanied by reflexive secrecy. Revelations of incompetence, such as the bungled investigation into the murderer Robert Napper, are invariably met with a closing of ranks. If individuals are disciplined for their failings, we do not hear about it.

Deficient leadership is, of course, responsible for these ills. The Metropolitan Police force was politicised by its previous commissioner, Sir Ian Blair. We await an answer to the question of whether his successor, Sir Paul Stephenson, will prove a reforming commissioner.

It is hard to see much impetus for change coming from the Government. Indeed, ministers seem determined to see the powers of the police grow, rather than diminish. A law potentially making it illegal for a member of the public to photograph a police officer came into force earlier this year. The person who took this video of Mr Tomlinson being attacked could, in a ghastly irony, be prosecuted themselves.

But the answer to the lack of police accountability, in the end, does have to be political. The London Mayor, Boris Johnson, showed that he is prepared to stand up to the powerful policing lobby when he demanded the resignation of Sir Ian last year. It is vital that Mr Johnson uses the powers available to him to bring the nation's principal force under control. And whichever party forms the next government should make police reform on a national level a priority.

The police are right to point out they do a difficult job. But this complaint misses the point. The days when officers could rely on the unquestioning trust and respect of the public are over. There has been too much misconduct, dissembling and secrecy over the years. Only an injection of accountability and wholesale reform can restore faith in those charged with delivering our safety.