The following correction was printed in the Guardian's Corrections and clarifications column, Tuesday 7 July 2009

The article below said that the three accused in the 1971 Oz magazine trial were acquitted. In fact they were sentenced to prison - overturned on appeal.

It started with Star Trek fans writing stories about a Kirk/Spock love affair, and it quickly became a craze. Fantasy fiction, or "fanfic" websites now attract contributions from large numbers of obsessive fans, and new genres are emerging at a remarkable rate: "slash" fanfic focuses on gay relationships (the Lord of the Rings characters provide particularly fertile ground), with "femslash" for lesbian characters; and then there's "real person popslash", where the unlucky subjects are celebrities in the music business.

One popslash fantasy came to public attention this week when, most unusually, its author found himself in court. Darryn Walker's writing is darker than most. The 35-year-old former civil servant's story, a 12-page article called "Girls (Scream) Aloud", depicted the kidnap, rape and murder of each member of girl band Girls Aloud by their coach driver.

The story was spotted by the Daily Star first, and then the Internet Watch Foundation, the internet regulatory body, which in turn notified the police. Walker's home was raided by Scotland Yard, and last October he was charged under the Obscene Publications Act - a 1959 law which hasn't been used against written material since the attempt to prosecute the publishers of Inside Linda Lovelace, a biography of a porn star, in 1976. The jury in that trial were unwisely told that if the book was not obscene, "nothing was" and showed their contempt for this argument by returning a verdict of not guilty. Shortly afterwards, the Williams report on obscenity and censorship recommended that similar cases should not be pursued in future.

To many, this was a victory for freedom of expression. "Lady Chatterley achieved freedom for great literature," says Geoffrey Robertson QC, who defended numerous notorious obscenity cases, including the trials of underground magazine Oz, with John Mortimer. "The Oz trials achieved freedom for not very great literature. And the Inside Linda Lovelace trial achieved final freedom for the written word."

Which, experts say, makes the decision to prosecute Walker for Girls (Scream) Aloud so surprising. "Ever since the Williams report, the notion of "obscene printed material is a contradiction in terms," says John Sutherland, a professor and expert on literary censorship and offensiveness who gave evidence in the Walker case.

Walker's fantasy about Girls Aloud was violent and graphic. "[The coach driver] uses blades and knives to mutilate their sexual organs, and a saw to dismember them while still alive, forcing them to perform sexual acts in a forlorn attempt to escape with their lives," wrote Don Grubin, a consultant psychiatrist who later gave evidence in the case. "The women are described as being sexually aroused in spite of, and indeed because of, their humiliation, pain and domination, responding sexually to their torturer and succumbing to his complete mastery and control of them. By any account the behaviour depicted in the story is extreme ... their body parts [are] sold on eBay."

But why prosecute, when there are so many graphic stories on the web? David Perry QC, the prosecuting barrister in the case, argued that: "[This article] was accessible to ... young people who were interested in a particular pop music group. It was this that distinguished this case from other material."

However, earlier this week the case was abandoned, when Walker's defence team produced evidence that Girls (Scream) Aloud did not pose a significant threat to the group's young fans.

"Darryn Walker's fantasy was not, in fact, easily accessible to the general public," the writer John Ozimek wrote in the Guardian shortly after Walker's acquittal. "A simple Google search for Girls Aloud will reveal millions of web pages dedicated to these individuals. You would need to focus the search terms much more closely (adding words such as 'rape' and 'murder') before you would be likely to chance upon this story and even then, we are still talking odds of one in 100,000," he said.

The sparse attempts to prosecute material since the 1979 Williams report confirms the fact that it is now very difficult to get results using existing obscenity law, experts say. The first ever CD prosecution, against rap group NWA, resulted in an acquittal at Redbridge magistrates court in 1991 when magistrates decided that tracks such as One Less Bitch or To Kill a Hooker, did not incite sexual violence, but simply reflected the depravity of street life in Los Angeles. Thirty thousand records, cassettes and CDs that had been seized by Scotland Yard's obscene publications squad were ordered to be released.

And the last high-profile attempt to achieve a conviction for written material failed in 1998 when the director of public prosecutions declined to prosecute Picador for publishing American Psycho, the Brett Easton Ellis novel depicting explicit killings of women.

Walker's case was the first attempt to prosecute internet fanfic, and it looks likely to be the last. "Most fiction remains fenced in by libel, copyright and other constraints," says Sutherland. "But fanfic is interesting because it is fiction without frontiers. And it is free."

The amount of potentially "obscene" material has grown enormously in recent years, as has the despondency amongst the authorities who have no hope of tracing or tackling the majority of it. A study in 2006 found that the UK has the world's fastest-growing market for online pornography, with 40% of men having visited porn sites the previous year, and a 30% rise in the number of women looking at porn online.

There has been a corresponding rise in the use of sado-masochistic and violent porn, with an American study showing that bondage and domination featured in 10%-20% of "mainstream" porn magazine covers. At the same time the number of prosecutions for obscene material has fallen dramatically. The government's own 2007 impact assessment of new laws against extreme porn, used the "very small number of proceedings" - an expected 30 a year - to justify the lack of additional cost of introducing new offences. The new director of public prosecutions, Keir Starmer, is widely expected to lead an era of further restraint, refraining from taking up cases where freedom of expression is an issue.

"The fact that [pornography] is so widely available and therefore completely uncontrollable by any single criminal justice system raises a serious question as to the legitimacy of selecting a single online author for prosecution - what are you achieving?" asks Tim Owen QC, the barrister who defended Walker. "Quite apart from the problem of defining what is obscene in legal terms, it is arbitrary and absurd from a policy point of view.

"On the one hand it is government policy to extend broadband access to every home in the country, while at the same time we all know that this enables one mouse click to the unpoliceable world of porn. The idea that, in a world where almost anyone can be an online publisher, you can use the 1959 legislation to police the written word on the internet is completely unrealistic."

But it is precisely this growth in internet porn which is leading some to call for tougher sanctions and more prosecutions, reflecting the wider divide about the appropriate response towards sexually violent material, especially where it is accessible to young people.

A recent YouGov survey found that 71% of all sexually active teenagers have viewed porn, with more than a quarter of teenage boys using porn at least once a week. The teenage girls I talked to during a recent Channel 4 programme, the Sex Education Show, said that boys were regularly asking them to perform "strange" sexual acts as a result. "Boys watch all this porn and then try to get us to do really weird things," one said.

Unless porn is specifically targeted at teenagers, the test for whether material is obscene is not how it would affect a teenager but the "likely" reader, deemed to be a porn-viewing adult. The issue that arises then, according to lawyers, is how to define obscene material, or material that merits legal sanctions.

The current test dates from 1868, when the then lord chief, Justice Cockburn, said: "I think the test of obscenity is this: whether the tendency of the matter charged as obscenity is to deprave and corrupt those whose minds are open to such immoral influences, and into whose hands a publication of this sort may fall."

The material deemed to fall into this category has changed radically over time. Case law is littered with now amusing examples of evolving attitudes, such as a decision in 1928 that Radclyffe Hall's novel The Well of Loneliness was illegal because of a scene where two women went to bed, that added "and that night they were not divided". The magistrate at the time felt those words would "glorify a horrible tendency".

Guidance for today's prosecutors states that action should only be taken where material has extreme or aggravating features, such as sexual acts with children, incest, bestiality, rape or mutilation. Similarly new laws which came into force earlier this year prohibit "extreme porn" for the first time. "Extreme" is defined as "grossly offensive, disgusting or otherwise of an obscene character".

But these definitions are still too woolly, many argue. "Terms such as 'indecent', 'obscene', and 'depraved and corrupt' should be recognised as too subjective to have any place in modern law and so should be scrapped," Julian Petley, professor and expert on censorship, wrote in the Guardian recently. "The only material that should be banned is that whose making can be proved to have involved the commission of illegal acts, such as non-consensual sex."

Even this is notoriously difficult to prove, as the Lovelace case showed. While the hardcore film Deep Throat was taken as a victory for women's sexual liberation at the time, it later emerged that Lovelace, real name Linda Boreman, had in fact been coerced into making it by a violent partner.

The relationship between porn and the exploitation of women has led to claims by some that "harm" caused should be viewed from a much wider perspective, an argument made in a well-known statement by Edwin Meese who, as the then US attorney general, conducted an inquiry into porn in 1986.

"Substantial exposure to sexually violent material leads to a greater acceptance of 'rape myth' in its broader sense," said Meese. "That women enjoy being coerced into sexual activity, that they enjoy being physically hurt in a sexual context, and that as a result a man who forces himself on a woman sexually is in fact merely acceding to the 'real' wishes of the woman."

The argument of a wider "social harm" still forms part of a lively debate about the ethics of tolerating material containing sexual violence.

"I would argue that the sexual free-for-all that pornography represents has caused serious harm in terms of the sexual health crisis, the rise in sexual offences and broken or unfaithful relationships, and the perpetuation of discrimination against women," says John Mayer, director of Mediawatch UK which campaigns for "decency" in the media. "Pornography has invaded every aspect of modern life, particularly television, film and the internet, to which there is unrestricted access. If you do not recognise these as 'harms' ... define your term."

Some experts are also dismissive of arguments that regulating pornographic material amounts to an unjustified infringement of freedom of expression, now protected in the UK under the Human Rights Act. "The freedom of expression of women may be circumscribed in a society which condones extreme pornography and in which their privacy is invaded by unwanted sexual violence or objectification," said Durham law professors Clare McGlynn and Erika Rackley, writing recently in legal publication the Criminal Law Review.

There have been several high-profile sex attacks linked directly to violent pornography, including the murders carried out by Frederick and Rosemary West and Peter Sutcliffe. In 2003 Brighton teacher Jane Longhurst was strangled by Graham Coutts, a 39-year-old obsessed with pornography showing strangulation, rape, murder and necrophilia, who then kept her body in a storage unit and continued to visit it for "sexual thrill".

"There was shock at the revelation that access to sites such as necrobabes and deathbyasphyxia is so easy," the Labour MP David Lepper said after her murder. "It provides access to the sort of material that fed Graham Coutts' fantasies - and it led to Jane's death."

The Longhurst case was one of the factors behind the new "extreme" pornography measures, which came into force in the Criminal Justice and Immigration Act earlier this year. But apart from the apparent intention to prosecute very few cases under this law, arguments about the impact of sexually violent porn on society are difficult to back up with evidence, psychiatrists say.

"While it may be that pornography can reinforce already existing sexual arousal patterns, I am not aware of any credible theory that simple exposure to pornographic images or the reading of pornographic literature is a relevant factor in the genesis of sexual deviance," says Don Grubin.

Grubin points to attempts to use pornography in a "therapeutic" context, to show its inefficiency in changing long-term patterns. "Attempts to change sexual arousal patterns in adults ... are notorious for their lack of long-term impact," he says. "The exception is the United States, where a relationship has been found between rape rates and the circulation of porn in different states, but this is stronger in respect of soft-core porn rather than the hard core porn similar to Girls (Scream) Aloud."

One of the effects of prosecuting cases such as Girls (Scream) Aloud which is clear, however, is the guaranteed perverse outcome that the publicity surrounding the proceedings draws attention to it on a massive scale.

"Inside Linda Lovelace had only sold a few thousand copies in the years before the 1976 court case," says Robertson. "Within three weeks of the case acquittal 600,000 copies were purchased by the public." Experts predict there will be a similar effect from the attempt to prosecute Walker. "The perverse thing about the Walker case is that everyone will read this Girls (Scream) Aloud now," says Sutherland.

And although the impracticality of Walker's prosecution attracted outrage amongst freedom of expression experts, they are slower to defend the material itself. "One of the problems with fanfic is that it is just so appallingly bad," says Sutherland. "In previous cases, such as Lady Chatterley's Lover, you could argue there was some redeeming social merit or literary quality. This has neither - it is rather unpleasant, sadistic fantasy.

"I think when they raided Walker's house the police were expecting to find a lot of horrific stuff on his computer," he added. "It just wasn't there. He was actually a rather dull man ... with the literary sensibility of a toilet seat. It's a pity a case was fought with such an objectionable and crass piece of fiction."

Offensive acts: A brief history of obscenity law

• 1590 Earliest conviction of man named Shaw for publishing advertisements for sex workers

• 1857 Parliament passes the Obscene Publications Act

• 1868 Benjamin Hicklin is prosecuted for the pamphlet, The Confessional Unmasked - Shewing the Depravity of the Romish Priesthood. The work contained "thoughts of the most impure and libidinous kind", the lord chief, Justice Cockburn, said, formulating the test of "tending to deprave and corrupt" applied to obscenity cases ever since

• 1953 Attempts are made to destroy copies of the Kinsey reports - academic studies of human sexual behaviour

• 1956 A number of publishers tried for "horrible tendencies" in their fiction

• 1959 Parliament passes an amended Obscene Publications Act, intended to strengthen the law but also "to provide for the protection of literature"

• 1960 Penguin acquitted for publishing DH Lawrence's 1928 novel Lady Chatterley's Lover. The prosecution is ridiculed for being out of touch after the prosecutor Mervyn Griffith-Jones asked whether it is the kind of book "you would wish your wife or servants to read"

• 1968 Hubert Selby is prosecuted for his frank portrayals of drug use, street violence, gang rape and homosexuality in his 1964 novel Last Exit to Brooklyn. The conviction is overturned on appeal

• 1971 Oz magazine in the dock for a schoolkids' edition, which included a highly sexualised Rupert Bear. "[This] case stands at the crossroads of our liberty, at the boundaries of our freedom to think and draw and write what we please," said John Mortimer, defending. The accused were acquitted

• 1976 The last jury trial for "obscene" written material for Inside Linda Lovelace returns a verdict of not guilty.