It's big, but it's really not clever ... JK Rowling launching Harry Potter and the Half-Blood Prince. Photograph: Chris Radburn/PA

It is time to make a stand against Harry Potter. A futile stand, no death or glory involved: just popping my head over the trenches so it can be mowed off by the vast, unstoppable juggernaut of popular acclaim before I have begun to open my mouth.

Firstly: if you're going to buy her book, don't buy it for half price at a supermarket. As an example of a world gone mad, you couldn't do much better than this: a writer whose sales have actually fulfilled a publisher's wildest dreams is indirectly responsible for large-scale misery among independent bookstores. This is not JK Rowling's fault. It's a consequence of the deregulation of the book market.

Tesco, say, sells the new Potter for about the same price as two tins of beans, in the hope that the mug(gle)s who do so will be convinced that everything in the shop must be marvellous value. Meanwhile, Mrs Betty Cardigan who runs the Lovely Bookshop Round the Corner has to grit her teeth and lose money every time she sells a copy if she is to compete, without being able to sell toilet paper or sushi to make up the difference. If you must buy the book, go to Mrs Cardigan's and, even if it is at a discount, insist on paying full price.

But whether you should buy the book at all is another matter. For I have come, with some regret, to this conclusion: their style is toxic. And this is Rowling's fault. I know that I am anticipating what the style of the latest book will be in advance of actually seeing it, but really, I don't think I'm going out on a limb here. Of course, if she has turned into a first-class writer with her forthcoming Potter book, I will happily, no, joyously, eat my words.

But until then, we have to swallow hers. And for all that she is gifted enough in devising popular scenarios, the words on the page are flat. I think it was Verlaine who said that he could never write a novel because he would have to write, at some point, something like "the count walked into the drawing-room" - not a scruple that can have bothered JK Rowling, who is happy enough writing the most pedestrian descriptive prose.

Here, from page 324 of The Order of the Phoenix, to give you a typical example, are six consecutive descriptions of the way people speak. "...said Snape maliciously," "... said Harry furiously", " ... he said glumly", "... said Hermione severely", "... said Ron indignantly", " ... said Hermione loftily". Do I need to explain why that is such second-rate writing?

If I do, then that means you're one of the many adults who don't have a problem with the retreat into infantilism that your willing immersion in the Potter books represents. It doesn't make you a bad or silly person. But if you have the patience to read it without noticing how plodding it is, then you are self-evidently someone on whom the possibilities of the English language are largely lost.

This is the kind of prose that reasonably intelligent nine-year-olds consider pretty hot stuff, if they're producing it themselves; for a highly-educated woman like Rowling to knock out the same kind of material is, shall we say, somewhat disappointing.

Children exposed to this kind of writing aren't learning anything new about words, or being stretched in any way; as Harold Bloom said, they're not going to be inspired to go off and read the Alice books, or any other enduring classic.

People go hoopla because they're delighted that Rowling has got children reading books - big, fat books without pictures at that. Can't argue with that: and maybe they will learn something about sheer reading stamina in the process. But it's all too easy.

The popular writer whose style is most similar is, it suddenly occurs to me, Jeffrey Archer (all those dead adverbs). All that paper, all those trees felled, all those words ... surely Rowling could have chosen some better ones, or put them together in a more exciting way?

She has, in her grasp, the power to galvanise minds instead of reeling out cliché after cliché. Will The Deathly Hallows do this? I hope so. But I fear not.