Take care. In reading this article, you may be in receipt of stolen goods. In fact, the organising committee for a certain upcoming sporting event has decided it would be "disproportionate" to prosecute the author of a book called Olympic Mind Games for breach of copy-right. But, under no less than two acts of parliament, it could if it wanted to.

When it discovered that Robert Ronson's children's science-fiction novel was to be published, the organising committee for the previously mentioned happening sent him an email asking that he should use neither the O-word nor the expressions "London 2012, or 2012 etc" in the title. The committee was able to do so under statutes passed in 1995 and 2006, which in effect turn all the elements of its title into a trademark.

In claiming copyright on a word, the organisation dedicated to the promotion of the competition to be held two years into the next decade is both following and extending a dangerous trend. As long ago as 1991, the official British artist of the first Gulf war, John Keane, faced protests and legal threats from the Disney corporation for having painted a picture of the devastation of a Kuwait beach which included a Mickey Mouse doll. Another British artist, David Haslam, faced legal action from the owners of the copyright on Noddy, and the American artist Rick Rush was taken to court for painting a picture of Tiger Woods. Both the Canadian mounted police and the Los Angeles police department sought to copyright their own logos, as OJ Simpson attempted to copyright his own name. A mock photograph in an art exhibition of Princess Diana and Dodi Fayed lookalikes cuddling a mixed-race baby was berated on the grounds of the infringement of copyright on her image. And Ofcom upheld long-haired, floppy-moustached 1970s athlete David Bedford's complaint against 118 118 for using two long-haired, floppy-moustached runners to advertise its directory inquiry service.

In several of these cases, corporations were acting to protect what they saw as their commercial interests. The email to Robert Ronson was written by the Olympic organising committee's manager of brand protection, concerned to "ensure that there was no confusion" as to whether the novel was "an official licenced product", presumably in case the committee seeks to declare Ian McEwan or Martin Amis official novelist to the 2012 Olympics at some point.

It's not just that the idea of copyrighting an entry in the English dictionary, or someone's face, haircut or name, is ridiculous. There is an issue of principle. By declaring images, titles and now words to be ownable brands, these various organisations and individuals are contributing to an increased commodification and thus privatisation of materials previously agreed to be in the public domain. For scientists, this constrains the use of public and published knowledge, up to and including the human genome. For artists, it implies that the only thing you can do with subject matter is to sell it.

As a consequence, people's view of what representation does becomes narrowly literal. Presumably, the Disney corporation felt that John Keane must have been either denigrating or exploiting its product when he used a doll on a beach to comment on the ironies of war. Similarly, painters, novelists and playwrights are attacked for representing Myra Hindley or Frederick West or James Bulger's murderers, on the grounds that to portray an action must be to promote it. Consulted by its British branch about the Olympic Mind Games case, the International Olympic Committee expressed two major concerns: that the word Olympic was used in the title of a work of fiction and that "there is no such thing as Olympic mind games". Clearly, the IOC hasn't grasped what the word "fiction" means.

Most expression involves reference to something real in the world. Most of our "experience" and indeed our "imagination" are formed from the image-making of others. Writers and other artists are rightly concerned about protecting their own copyright, but they should be equally concerned with the shrinking of the public domain. Ronson's refusal to be cowed into changing the title of his novel is a victory for the idea that there is more to free expression than the right to advertise.

· David Edgar is a playwright and president of the Writers' Guild DavidEdgar@blueyonder.co.uk