The rotten apples argument has served the police well for as long as I can remember, because everyone knows that any large group of people contains a few individuals who abuse their position and bring shame on the vast majority of their hard-working colleagues. But most forget that the point of the barrel of apples metaphor is that the rot spreads to all the apples.

There has never been a more graphic proof of this than in the Plebgate case, when police officers of varying rank lied to destroy a democratically elected politician's career to increase political leverage in a time of cuts. We must now consider that the rot has spread, that the police service in England and Wales is so infected by a culture of dishonesty, expediency and outright corruption that radical reform is now the only option.

What was striking about the Andrew Mitchell case, quite apart from the institutional vindictiveness it betrayed, is that police were so used to seeing their lies prevail in court that they did not even bother to lie very well about the incident in which they alleged he used the word "pleb". They were caught out by CCTV coverage and a recording of a subsequent meeting between the Police Federation representatives and Mr Mitchell.

Yet within a few hours of condemning these actions, David Cameron was telling a Police Federation event: "Britain has the bravest and best force anywhere in the world." Normal service had apparently resumed and officers no doubt left the event satisfied that the whole shoddy edifice of 43 constabularies was not going to be dismantled, nor inconvenienced by deeper questions about the decline in ethical standards of the police.

But this is not about just one case. Since the last election, there has been a slew of really shocking stories, which cover the whole range of police activity. In front of me I have a 330-page file, dealing with scores of reported cases, which was assembled by a research assistant using the web over the summer. It is not by any means a scientific survey and we are sure it is not yet complete, but I have to say that it gives a much better picture of recent developments in the character and practices of the modern police than anything I have seen produced by the political parties or indeed the Independent Police Complaints Commission.

Collectively, these newspaper articles that involve the fabrication or destruction of evidence, deaths in police custody, framing of suspects, abuse of databases for personal reasons, punitive attitudes to innocent members of the public, abuse of the Taser stun gun, contempt for legitimate protest, examples of racism and use of excessive force and restraint provide a much more accurate portrait of the police than is generally available.

I concede that we could easily compile a list of heroic actions by police as well as examples of hard work, brilliant detection, compassion and so forth. That's part of the picture too, as I realised when I had dealings with a policeman who broke down a door to bring help to a friend who was dangerously ill and then went to the bother of finding my home address. We all have stories like that, but this is what we expect of police officers and in no way do they balance or mitigate the problems we're seeing. Nor do they deter the growing belief that we need a royal commission to look into the structure of the police, training, ethical standards, investigation of wrongdoing and ways of improving the currently sketchy record of the Independent Police Complaints Commission.

None of us has much time to pay attention, so we are only dimly aware of these stories. The alarm bells do not go off when we read of missing boxes of evidence that would have helped prosecute eight police officers in South Wales for framing an innocent man, which turn up too late; or of the delays in release of video evidence, which disproved a violent disorder charge against a peacefully protesting student. We shrug when a senior police officer admits to inadvertently misleading a House of Commons committee to the effect that the legitimate demonstrators were not monitored by undercover officers, when in fact they were, or hear of a four-year corruption inquiry in Enfield that concluded with nine officers found guilty of discreditable conduct, yet with no dismissals or senior officers punished. No systemic failure, we are told.

We forget about the abuse of police power that led Cleveland officers to arrest and falsely imprison a leading local defence lawyer. We struggle to know what to think about the deaths in police custody of Sean Rigg, Olaseni Lewis and Jake Michael, the last of whom was said to have suffered a cocaine-induced "excited delirium" after being restrained by up to 11 officers and the use of pepper spray. What are we to believe when we read that unarmed Anthony Grainger was shot dead while in his car in a Cheshire car park? Did he really represent a threat? And was Mark Duggan really lawfully killed?

The cumulative effect of these recent stories, plus the scandal of Andrew Mitchell's nightmarish case, makes us all far less likely to believe the word of a police officer, which is the position many lawyers who work on criminal cases in the magistrates' and crown courts reached several years ago. It is an accepted part of their professional lives that clients are sometimes fitted up and that evidence and statements are finessed to suit the police account of an incident, often to the effect that the police were assaulted by a defendant. There's not a lot they can do about it.

Even when police officers are caught in serious wrongdoing, they are usually allowed to resign (on full pension) rather than face disciplinary hearings. The crimes and misdemeanours of the police are easily buried.

If you talk to the solicitor Iain Gould who has a national practice that brings cases against the police, he'll tell you that a lot of his clients won't bother with the IPCC's complaints procedure because they know their grievance will fail at an early stage. So his clients opt to go straight to civil law, at which point the police make a "without prejudice" offer to settle and the details of the case are never heard in open court. "There is a culture of impunity," he told me."A pervasive attitude that they can do what they want and get on with it."

That is a terrifying assessment but at least the politicians now know it can happen to them as easily as an ordinary person in the street. Time for that royal commission.