Next week Trenton Oldfield, the campaigner who disrupted this year's Oxford and Cambridge boat race, will be sentenced for causing a public nuisance. This catch-all common law charge allows the courts to impose any sentence up to life imprisonment.

The original charge was under section five of the Public Order Act. A conviction for disorderly behaviour can be imposed by magistrates and carries a maximum fine of £1,000. But after media and political pressure, the charge was changed.

So for disrupting the boat race and causing injury to nobody (the person most at risk was himself) it is possible that Trenton Oldfield may share a similar sentence to a man who made 1,000 obscene calls to women over the space of two weeks, and was jailed for nine years; to a man who persistently stalked a female police officer and got 30 months; or to somebody who obsessively made hoax 999 calls, and was eventually jailed for eight years.

Previous convictions for public nuisance suggest that significant factors in sentencing should include either the intent to cause harm or recklessness about the consequences of one's actions. Disrupting a sporting event might be inconvenient and antisocial, but it's neither harmful nor malicious.

But if public nuisance legislation is to be used as a way of stifling protest we are in very different territory. It turns the judiciary into a political tool to be used against a government's opponents, outlawing opposition that doesn't use approved channels.

The UK has a long history of allowing public spaces to be used for all sorts of campaigns and demonstrations. Most have passed without incident; others have caused major inconvenience. For the most part, those who take part are allowed to make their point.

At the turn of the millennium hundreds of lorry drivers and farmers took part in one of the most disruptive protests this country has seen, blockading fuel depots and restricting supplies to hospitals and supermarkets. The Institute of Directors put the costs at more than £1 billion. Yet William Hague, leader of the Conservative Party at the time, called the protesters "fine, upstanding citizens". Nobody was jailed for the disruption they caused or for the danger they might have caused.

So why should Trenton Oldfield be singled out? You could argue that he spoiled a lot of people's fun. But some would say that the ban on smoking in public places has spoiled a lot of people's fun. Most weekends in Britain the rain spoils a lot of people's fun. We live with it.

You could argue that his protest offended the establishment. It clearly achieved that, even if it was unlikely to achieve much else. But we have TV shows and newspapers that lampoon the establishment every day. They don't get prosecuted.

You could argue, as the prosecutors did, that Trenton Oldfield was putting himself and possibly the rowers at risk. But that risk was minimal and no harm was done. If it is now the job of the courts to prosecute people for what might have been, then all our lucky escapes may come back to haunt us.

So it seems the law of public nuisance is being used selectively and disproportionately in this instance to criminalise protest - and only two years after the Law Commission mooted proposals to simplify or abolish the offence.

It doesn't matter whether you approve or disapprove of this particular protest, any more than it matters whether you think the lorry drivers were right to make their point about fuel prices or not. A healthy democracy champions the right to dissent, and to do so visibly and publicly. If protests cause harm, then judges have all the artillery of criminal law at their disposal; if no harm is done, they should leave well alone.

The irony of the story is that if the police had simply sent Trenton Oldfield away with a caution, the incident might have been quickly forgotten, appearing perhaps as an obscure pub quiz question in years to come. As it is, the courts are likely to give his cause more publicity than he can ever have hoped.

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