Confession and repentance are not among the Christian virtues practised by the pope. He has apologised for the rape of children by Catholic priests in Ireland; but this is one of the few paedophilia scandals now shaking the church in which neither he nor members of his inner circle were involved. He condemned the Irish bishops' "grave errors of judgment" and "failures of leadership", but of his own grave errors and failures – in Munich, Wisconsin and California – he says not a word, except to dismiss the issue as "petty gossip". His response to this scandal reminds you of the origins of the verb to pontificate.

Shut out of his closed, self-regulated world, the victims of sacerdotal rape could only rage in frustration. Until now.

Over the weekend, the authors Richard Dawkins and Christopher Hitchens announced that they've asked lawyers to prepare a case against the pope. Recently, in the Guardian, Geoffrey Robertson, the barrister they are consulting, explained that senior churchmen who protected paedophile priests, swore their victims to secrecy and allowed the perpetrators to continue working with children, committed the offence of aiding and abetting sex with minors. Practised on a large scale, this becomes a crime against humanity recognised by the international criminal court. This is the general Vatican policy over which the then Cardinal Ratzinger is accused of presiding. When Benedict comes to the UK in September he could, if Dawkins and Hitchens get their warrant, be arrested.

At last we are waking up to what international law means. For the first time in modern history the underlying assumption of political life – that those who exercise power over us will not be judged by the same legal and moral norms as common citizens – is beginning to crack.

International law is the belated reply to one of the oldest surviving aphorisms in the English language. There are half a dozen versions, but the best known is this: "They hang the man and flog the woman / That steals the goose from off the common / But let the greater villain loose / That steals the common from the goose." This is the way we thought it would remain. The powerful were licensed by our expectations to carry on committing great crimes, while their subjects were punished for lesser offences. No longer. Picture the pope awaiting trial in a British prison, and you begin to grasp the implications of the radical idea that has never yet been applied: equality before the law.

At the same time as Dawkins and Hitchens laid out their case, the barrister Polly Higgins challenged our perceptions of what legal equality means. Last Friday she launched a campaign to have a fifth crime against peace recognised by the international criminal court. The crime is ecocide: the destruction of the natural world.

The laws of most nations protect property fiercely, the individual capriciously, and society scarcely at all. A single murder is prosecuted; mass murder is the legitimate business of states. Only when these acts are given names – genocide, crimes against humanity, war crimes, crimes of aggression – do we begin to understand their moral significance.

The same applies to nature. The Wildlife and Countryside Act 1981 criminalises anyone who "intentionally picks" a single flower from a protected plant. But you can grub up as many as you like as long as it's "an incidental result of a lawful operation". Pick a buttonhole and you could find yourself in the dock. Plough out the whole habitat and the law can't touch you.

Higgins gives some examples of ecocide: the tar sands mining in Alberta, the Pacific garbage patch, the pollution of the Niger Delta by oil companies. She points out that ecocide is rarely a crime of intent, but in most cases an incidental consequence of other policies.

Company directors or politicians could be prosecuted individually, but instead of being fined they would be charged for the restoration of the natural systems they've damaged. The purpose of criminalising ecocide is to raise the costs of trashing the planet to the point at which it ceases to be worthwhile. This is the obvious outcome of a wider understanding of legal equality: why should private property be protected while the common wealth of humanity is not?

International law as currently applied is often described as victors' justice: the only people who get prosecuted are those who lose the wars they fight with powerful states. It's not even that. Last week we learned that about 50 suspected war criminals or human rights abusers are living in Britain. Among them are alleged torturers who worked for Saddam Hussein's government, one of Robert Mugabe's henchman, a member of Sudan's Janjaweed militia and a gruesome collection of Afghan warlords. But the police have been given no budget to investigate them and the Crown Prosecution Service has no resources with which to pursue them. So, while shoplifters are sent down, alleged mass murderers walk freely among us.

So much for the prime minister's promises. A month ago, after Tzipi Livni, the former Israeli foreign minister, cancelled her visit to Britain for fear of being arrested under a warrant obtained by human rights campaigners, Gordon Brown wrote an article for the Telegraph in which he proposed to stop private prosecutions for crimes against humanity. Brown maintained that the warrant was supported by only "the slightest of evidence", and that those seeking Livni's arrest had "set out only to grab headlines". But the evidence for the crimes against humanity to which Livni has been linked – laid out in the Goldstone report and elsewhere – is massive, detailed and hard to dispute.

Brown went on to make another statement that was plainly false: "Britain will always honour its commitment to international justice. The police here remain ready to investigate cases; the Crown Prosecution Service to bring them; the courts to hear them." His government has rebuffed calls to set up a specialist war crimes unit and failed to produce a dedicated penny for the prosecution of war crimes suspects.

Then he explained his real purpose in seeking to prevent private actions. People like Livni, he said, represent "countries and interests with which the UK must engage if we are not only to defend our national interest but maintain and extend an influence for good across the globe". Britain, in other words, will not investigate or prosecute its allies. His article demonstrated the opposite of what he set out to show: that if there is a case for prosecuting foreign dignitaries visiting this country, the authorities will take care of it. Without private actions of the kind that Dawkins and Hitchens hope to launch, equality before the law remains an empty threat.

Brown's desperate wriggling over the Livni case suggests that governments are beginning to grasp the shocking implications of what they have signed up to. It's time we did the same. There's a promise implicit in international law: the end of the age of exceptions.

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