THE SHAME once associated with looking at dirty pictures has fallen away since porn moved into the mainstream, around the time of Madonna's Sex book. With the rise of the internet there can be few adults in the UK who have not seen some porn, somewhere.

But all of these people could soon, without knowing it, be breaking the law. This week, with little fanfare or media debate, the Labour government finally puts on to the statute book its Criminal Justice and Immigration bill, which creates a new offence of possessing "extreme pornography". The bill is based on a joint Home Office/Scottish Executive consultation on the possession of extreme pornographic material undertaken in 2005/06, and though this bill is for England and Wales, the law in Scotland is likely to follow suit.

So what could possibly be wrong with banning something as forbidding as "extreme pornography"? First, it's unworkable. Second, it will be ineffective in targeting the genuine problem of sexual violence in society. And third, it's undesirable in itself because it reverses an important liberal trend of recent times - society's acknowledgement of the right of consenting adults to indulge their private desires and fantasies without the interference of the state, as long as they do so without harming others.

Why is it unworkable? "Extreme pornography" is defined in the new legislation as images which portray "in an explicit and realistic way" acts that "threaten a person's life", or that could likely result in "serious injury to a person's anus, breasts or genitals". Possession of such images will now be a criminal act. The Obscene Publications Act already imposes criminal liability on the publishers of material likely to deprave and corrupt. What is being outlawed here, for the first time, is the possession of images. Illegal images will include portrayals of acts which are not in themselves illegal, such as bondage sessions between consenting couples.

"Explicit", "realistic", "threatening" and "serious" are hugely subjective terms, with different meanings to different people. Much of the pornography which circulates these days is of the S&M-themed variety - images of people being tied up, spanked, tickled with feathers. For some, such images herald the end of civilisation as we know it. For others, it's the spice of life.

Fetishistic images are commonplace, not just in porn but in the worlds of advertising and art, where a bit of stylishly-photographed bondage, leavened with some postmodern irony, is virtually guaranteed to shift product. In Madonna's Sex book there's a fair sprinkling of "extreme" imagery, including lesbian sadomasochism, images of rape, even a hint of (albeit playful) bestiality. Consider too, Ang Lee's Lust, Caution, just released on DVD after a successful box office run, which might fall foul of a literal interpretation of the new legislation.

The photography of Nobuyoshi Araki frequently features his female subjects bound and trussed (although his models are wealthy volunteers who have paid for the privilege). Robert Mapplethorpe's bloody images of homosexual S&M have been shown in prestigious galleries all over the world.

These are precisely the kinds of images, depicting consenting adults engaged in deviant but legal acts which, if found in your possession by an overzealous bobby, could henceforth bring down a three-year prison term and a place on the sex offenders' register.

And what if you would never dream of having such images in your possession, but accidentally downloaded a few by Googling "water sports" or opening a spam email from a "lusty lady"? Supporters of the legislation scoff at such hypotheticals, and insist that it will not be applied unreasonably. It may be paranoia to think that the police will want to waste much time chasing down the nation's adult bondage fetishists in 2008. The point is, if they want to they can.

At the third reading of the bill in the Lords last week, Baroness Miller eloquently put the case against: "The legislation allows the police to pick up someone watching this material before they commit any crime, before they actually cause any harm to another person. Often they may be arresting somebody who has strange tastes, ones we might find repugnant, but who is never going to harm anyone else and is no threat to society That is truly the domain of the thought police. These clauses are the State entering the bedrooms and minds of citizens before they commit any crime that involves harm to another human being."

Assuming that a workable definition of "extreme pornography" can be reached, banning it will in any case be ineffective in dealing with the very real problem of sexual violence in society. Like so many bad laws and ill-considered bans, this one comes out of knee-jerk reaction to tragedy. In 2003, a teacher from Brighton was murdered. The killer, it emerged in the trial, had been a user of websites specialising in sexually violent imagery. The victim's mother embarked on a campaign to outlaw such usage, which was taken up by then-Home Secretary David Blunkett.

There is no conclusive evidence to support the suggestion of a causal link between consumption of pornography and the committing of sexual violence. The Scottish Executive concedes this in its 2005 consultation document. When this proposal was first mooted by the government, criminologist David Wilson observed that it was "based on the idea that viewing violent images produces violent acts. However, there is now 60 years' worth of research that suggests this simply isn't the case."

Peter Sutcliffe cited the Bible to justify his murders of prostitutes in Leeds. Fred and Rosemary West used pornography. Was the Bible to blame for the Yorkshire Ripper, or pornography for the horrors of Cromwell Street? If only the cruelties of some human beings were so easily explained, then they might be prevented.

I also said the new law was undesirable. Those societies where porn is most freely available, and least stigmatised, such as Denmark, Sweden, Holland, Germany, are also those where women's and gay rights are most advanced. Those where it is most savagely censored, such as Iran and Saudi Arabia, are where women still live in feudal conditions, and homosexuality is punishable by death.

I claim no simple cause and effect link here, but merely note the parallels. A relaxed attitude to porn, progressive sexual politics, and tolerance of others - even those who like a bit of rough now and again - frequently go together. Tony Blair, notwithstanding his private religious convictions, understood that when he said: "It is not for the state to tell people that they cannot choose a different lifestyle, for example in issues to do with sexuality". It's a pity his successors in government seem to have forgotten that principle.

Brian McNair is professor of journalism and communication at the university of Strathclyde, and the author of Striptease Culture (Routledge, 2002) and Mediated Sex (Arnold, 1996)