Hundreds of thousands of us have now seen the footage of the newspaper-seller shambling peacefully home from work. We've seen how, without warning or provocation, PC Simon Harwood attacked him from behind, hitting him with a baton then shoving him to the ground. We know that the officer had unlawfully removed his badge, and that his face was obscured by a balaclava. We know that, a few minutes afterwards, Ian Tomlinson collapsed and died. We also know that the Metropolitan police lied about his death to the media and to Tomlinson's family.

Fifteen months later the director of public prosecutions, Keir Starmer, decides that "there is no realistic prospect of a conviction against [Harwood] for any offence arising from the matter investigated and that no charges should be brought against him". The evidence for his role in Tomlinson's death, Starmer says, is contradictory, and the time limit for pressing lesser charges has sadly expired. Starmer provides no convincing explanation of why it has taken him so long to make his decision, or of why a jury should not be allowed to make its own assessment of the evidence.

Now picture the opposite case: a civilian launching an unprovoked attack on a policeman, captured on film, which is immediately followed by the policeman's death. The Crown Prosecution Service ponders and dithers before deciding that the assailant should get away scot free. Implausible? You have just understood that in the United Kingdom equality before the law exists only in textbooks.

The excuse Starmer gave is that conflicting medical evidence means that a causal link between the assault on Tomlinson and his death could not be proved beyond reasonable doubt. It is true that the evidence is not consistent. But nor is its quality.

Starmer's decision not to prosecute PC Harwood rests on the first autopsy, conducted by the pathologist Dr Freddy Patel. Other pathologists have expressed astonishment that Dr Patel was chosen for this case. A Home Office standards committee had already ruled that he had not maintained professional standards in three other cases, after he had failed to detect what appeared to be clear evidence of injuries. He is facing a disciplinary hearing before the General Medical Council for alleged incompetence in 26 cases.

This isn't the first run-in he has had with the council. In 1999 he was reprimanded by the GMC for speaking to reporters about the death of a man in police custody that he was investigating, and making an unsupported allegation against him. It looked like an unwarranted attempt to help the police out of a tricky situation.

Patel decided that Tomlinson died naturally. But he found three litres of fluid in Tomlinson's abdominal cavity. His notes initially suggest that this was blood. He disposed of the fluid. Then he changed his notes to suggest that it wasn't blood but something else. Two subsequent postmortems, conducted by far more eminent pathologists, both concluded that Tomlinson died of internal bleeding consistent with his body hitting the pavement.

We don't yet know why Patel was chosen to conduct the first autopsy, but it is widely believed he was recommended to the coroner by the City of London police. The police have refused to comment. Why could a jury not have been allowed to decide which autopsy, and which pathologists, it trusted?

This is a moment in which the pomp and majesty of the law falls away to reveal a squalid little stitch-up. In years to come you will hear Keir Starmer's decision mentioned alongside the Widgery report, the Hutton report and the failure to prosecute the killers of Blair Peach and Jean Charles de Menezes. The Tomlinson whitewash will be seen as one of British officialdom's most notorious swindles.

The difference in this case is that, thanks to citizen journalism and the Guardian's investigation, we have unequivocal footage of what happened to the victim. We also know that the Independent Police Complaints Commission, which made its own assessment of the evidence, found that the case was strong enough to warrant prosecuting PC Harwood for manslaughter. This time the Crown Prosecution Service cannot hide behind police lies about what happened. We know what happened: we've seen it. This makes the stitch-up even more infuriating and obscene.

So what can be done? Ian Tomlinson's family doesn't have the money to mount a private prosecution. But could we not raise it for them? I would welcome some advice about how much would be needed and how it could best be found. But right now our duty as citizens is to raise Cain: to show that we will not accept such blatant inequality before the law. If not now, when?