It is a triumph and reassurance for all authors that J.K. Rowling has won her plagiarism case in New York. And it is a tribute to her fortitude and integrity that she took it on in the first place.

My support is not some kind of blind solidarity between authors, or the result of a dog-in-the-manger mentality about the use of our work.

Let me explain. Since he first burst upon the world 11 years ago, Harry Potter has sold in book form more than 400 million copies. The reason for reiterating this isn't to stun you yet again, but to point out that such a degree of visibility and success inevitably attracts hordes of imitators, plagiarists and idle opportunists.

This is exactly what has happened. Jo Rowling has been plagued by people trying to scramble aboard the Harry Potter juggernaut, and has, rightly in my view, been admired for her tolerance and good-humoured resignation to this being one of the unavoidable consequences of success.

Companion books (Harry Potter and the Torah) and parodies (Hairy Porter and the Marijuana Stone) abound, and her only (graceful) act of remonstrance has been to write two companion volumes herself, both for charity, that have so far raised £16 million between them.

So far, so patient. But even the most forbearing of worms will turn. The final straw was a Harry Potter “lexicon”, written by a so-called fan in America, Steve van der Ark.

This “lexicon” (the word actually means a vocabulary to be used in conjunction with a particular subject) consists of lifting the names, situations and plotlines in the novels, overwhelmingly in Rowling's own words, putting them in alphabetical order, and selling the result as if it was the fan's own work. As the judge said, there are almost no extra insights, no extra information, hardly even the courtesy of quotation marks. Van der Ark has simply taken someone else's hard work and rearranged it, badly, as his own.

This is not a matter of that age-old - and impossible - difficulty of the plagiarism of ideas. It is something much easier to define, and a danger to all writers. It is - let's not mince our words - the theft of someone's writing, someone's own words stolen in exactly the form in which their brain produced them. And it's a theft to which all writers are vulnerable.

Jo Rowling didn't have to do this. I should think that her time in New York was horrible, exacerbated by a lack of support caused, no doubt, by deeply unattractive sourness over her wealth. Well, I applaud her, and I bet I'm not alone. I am thrilled for her, and very grateful to her, for taking the stand she did - and winning.

Joanna Trollope's latest novel is Friday Nights (Bloomsbury)

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