Watch out! There's an intellectual thief about ...

Accusations of copying in pop, film and fiction are on the increase. Are there just not enough new ideas to go around?

Where there's a hit, there's a writ. Cute phrase, though it's not the first time it has appeared in print. However, let's not allow that to trouble us here, as the subject on the card is plagiarism - the act of appropriating someone else's work or ideas without giving them the credit. On Monday, Loudon Wainwright III - the singer-songwriter whom you might remember performing guitar interludes on M*A*S*H - took on the might of pop demigod and former Take That boy, Robbie Williams. Robbie, Wainwright alleges, has been caught with his hand in the terpsichorean cookie jar. Williams's song "Jesus in a Camper Van" - a track on his platinum-selling album I've Been Expecting You - is not, it seems, all his own work.

Williams's lyric includes the lines, "Even the Son of God gets it hard sometimes/ Especially when he goes around saying I am the way". Back in 1973, LW III sang something supernaturally similar on his record, "I am the Way": "Every son of God gets a little hard luck sometimes especially when he goes around saying he's the way." Williams's record company EMI aren't denying that the words are Wainwright's, as they credited him on the album cover. It's just that they failed to get his permission first.

If Wainwright is successful in his action, precedents suggest that he could be in for a generous slice of Williams's royalties. George Harrison was forced to give up a percentage of his earnings to The Chiffons, who accused him of duplicating the melody of their 1961 hit "He's So Fine" in his 1970 hit "My Sweet Lord" - despite the judge's ruling that the plagiarism had been unconscious.

The Rolling Stones topped up their overseas bank accounts when they successfully took action against The Verve for using a sample of their 1965 track "The Last Time" on the younger band's hugely popular "Bitter Sweet Symphony". As Robbie Williams earned £7.5m last year without even releasing an album, Wainwright may be able to touch him for enough cash to keep him in plectra for the rest of his life.

It's been a juicy couple of weeks for the intellectual-property lawyers. On 17 March, JK Rowling was sued by Nancy Stouffer, an American author who alleges that one of her stories furnished many important details of the Harry Potter series - a character called Larry Potter, for a start. Stouffer's case is a multimillion -dollar affair, as she's also taken arms against Time Warner, the film company about to launch a lavish film adaptation of Rowling's work.

And it doesn't happen solely to household names. The American poet and academic Neal Bowers became a reluctant authority on literary piracy when he discovered that versions of his published work were resurfacing in literary magazines, credited to a David Jones. One factor made this act of theft difficult to forgive: the stolen poems were elegies for Bowers' late father, so the plagiarist had not only stolen the poet's work, but colonised his experience of bereavement.

Some of his colleagues - particularly those who subscribed to the once-groovy post-modern theory that the text is constituted by the reader's experience of consuming it - couldn't see what the fuss was about. Bowers disagreed. "I'm one of those old-fashioned people who believes in right and wrong, and honour," he explains. "The idea that a text is community property is sensible only to the extent that it indicates a dynamic relationship between writer and reader. When the notion is driven to the absurd conclusion that a text exists only when read and is therefore the property of the reader, I quickly lose my patience."

And once his patience had been lost, he put a lawyer and a private detective on the case to track down and prosecute his textual stalker. The trail led to a convicted child-molester and former primary-school teacher whose acts of plagiarism were, in some way, an expression of his disordered mind. "The pursuit of a plagiarist rarely concludes satisfactorily," Bowers says. "In my own case, I finally abandoned my efforts to hold David Jones accountable. The chase became too costly, financially and emotionally. In the end, I wrote a book about the matter [entitled Words for the Taking] as a way of putting the whole thing to rest, but I suspect that I gave Mr Jones the very kind of notoriety he desired."

In Bowers' case, the culpability of the plagiarist was clear. Other examples demonstrate the difficulty of distinguishing robbery from coincidence. You'll probably have seen The Blair Witch Project, the no-budget movie in which a gang of young filmmakers get lost in the woods on a hunt for a legendary spook - and soon all that's left of them are a few reels of videotape. You may well not have seen The Last Broadcast, a no-budget movie which the same synopsis would fit.

It was the online magazine 11th Hour which first examined these films, produced an exhaustive inventory of uncanny similarities, and showed that in everything from shooting dates to the establishment of a fake factual website, The Last Broadcast got there first. "There's definitely something going on," asserts Stefan Avalos, director of The Last Broadcast. "There was definitely influence. But I don't think there was all-out theft." He does, however, wonder why Blair Witch backer John Pierson - who knew that The Last Broadcast was in production before Blair Witch started shooting - has been so keen to rubbish the earlier work.

If Avalos had issued a charge of plagiarism, he could have ended up with ectoplasm on his face: such cases are notoriously difficult to prove, and are often settled in out-of-court deals. In November 1997, an obscure talent named Barbara Chase-Riboud accused Steven Spielberg's Dreamworks company of strip-mining her novel Echo of Lions to provide material for the movie Amistad. Spielberg's lawyers retorted by suggesting that Chase-Riboud's book had ripped off an earlier work, Black Mutiny by William Owens. In February 1998, Chase-Riboud suddenly changed her story: "I think Amistad is a splendid piece of work," she whooped, "and I applaud Mr Spielberg for having the courage to make it." Nobody would say whether any money had changed hands.

Perhaps what these cases really prove is what structuralist literary critics have been telling us for years - there are only a certain number of narrative components in the storyteller's bran-tub. And this may be why so many creative people - Ridley Scott, Ben Okri, PD James, Graham Swift and the Spice Girls to name a few - have found themselves accused of intellectual theft.

So, writs continue to be issued, and legal types grow fat on the proceeds - George Harrison's case, for instance, kept a gang of lawyers occupied for 20 years. Bowers hears news of such cases with regret, "because it's so difficult for the wronged person to find justice. Even if he wins a settlement, a sense of violation lingers".