I arrived late and in a rush, as usual, to find the Bestselling Author already tucking in to his amuse-bouche in the sumptuous private dining room, just off Piccadilly. Lord Byron scowled down handsomely from the wall, candlelight flickered on silver cutlery. Still slick with rain, I slid into my chair next to the BA. I felt bad about being late, felt like a troublemaker generally - we were gathered to have a Victorian dinner in honour of his rip-roaring pastiche novel of sensation, and, as a vegetarian, I wasn't going to eat any of the carefully created 19th-century delicacies.

"Oh, don't worry," said the BA. "My wife's a vegetarian, too." He indicated a blonde sitting at the end of the long table. At the very moment I turned to look, she was lifting an oyster shell to her mouth and chugging down the contents. There was a brief, uncomfortable pause before I protested, "But she's eating an oyster!".

"Oh, she eats fish," he said, calmly.

I took a deep breath. "Then she isn't a vegetarian," I said, very firmly. "And she shouldn't call herself one."

I'm what you might call a late-onset vegetarian, having become one only two years ago. I've always been a foodie, proudly jointing my £8 organic free-range chicken to get three meals for two people, and insisting on "happy pig" chops. Then one day, with the chance purchase of Madhur Jaffrey's epic tome, World Vegetarian, it all just stopped.

At first, I didn't worry too much about the odd prawn slipping down - as a literary editor, much of my diet consists of book-launch canapés - but gradually I had to admit that they tasted more and more horrible. There have been bumps and surprises along the way, mostly courtesy of my family: the aunt who said, "I could give up vegetables completely, but meat, no way. I must have meat!", so intensely that I started backing away - if someone didn't give her a sausage quickly, she might start on my arm. Another family member, an ex-domestic science teacher, took to asking, "How are you getting on with your low-protein diet?". But the biggest surprise is that saying, "I'm a vegetarian", is invariably followed by the question, "But you eat fish, don't you?".

"Oh, wow, you're really hardcore then, aren't you?" was one response to my negative. "Oh, you're a real vegetarian," is the more usual comment. While I loved the application of the word "hardcore" to my lifestyle (that was a first), I have to ask: what is so vegetarian about fish? They have eyes! They have brains and spinal columns! They have nothing in common with leeks!

If anything, I feel stronger about the crazy depletion of fish stocks, the rape of the sea, than factory farming - and I feel pretty bad about factory farming. One eureka moment came when I saw a photograph of a net full of deep-sea fish. Every mouth was strained open, and extruding from their sockets, like toothpaste from a tube, were the whites of their eyes, due to the lack of water pressure. A more potent image of agony is hard to imagine, and the utterly unbothered look on the face of the fisherman in the shot is haunting.

Still, fishcotarianism is rampant. At a buffet dinner with two old friends, both lifelong vegetarians - or so I thought - I was helping myself to pasta salad when - whoops! - a sliver of salmon slid out. "Watch out," I cautioned John, "that one's got salmon in it." "Oh, we love salmon... Darling! Salmon over here!" he called to his wife, who got stuck in. The last three "vegetarians" I met all trilled, "Oh, I'm a veggie... but I eat fish".

Then there's the Thai restaurant that boasts a "vegetarian and vegan menu" (to add insult to injury) featuring sea-bass and prawn. And the garden party I went to in the summer, with canapés of rare roast beef, mini burgers, stuffed new potatoes topped with fish roe and mini fish-and-chips. When, in desperation, I asked for the vegetarian choices, I was waved towards a platter of smoked salmon. And the lunch in a friend's garden when I was offered a lavish selection of seafood. "Well, I know you're a vegetarian, but I thought you ate fish!"

Now, I know that someone who was still eating steaks two years ago can't commandeer the moral high ground. My demi-veggie friends were virtuously avoiding meat when I was still chowing down on bangers. But the Vegetarian Society is taking this so seriously that they have begun a campaign on the issue. Their latest members' magazine features a picture of a far from unusual sight: a canteen sign saying, "Vegetarian special: fishcake". One member reported being offered a fish dish in a restaurant, only to be told, angrily, when she refused it, "My sister's a vegetarian, and she eats fish".

The society has produced a series of cards to be handed out to those "who don't seem to realise that fish are animals". "Don't be a pesky-vore by ordering fish and calling yourself a veggie, or offering fish to your vegetarian guests or customers", it chides, contrasting a fish with a carrot: "Taxonomic classification - Animal. Suitable for Vegetarians - No".

So, why is this myth so prevalent? Don't get me wrong, anyone who cuts down their consumption of animal products is to be applauded, and it's up to the individual conscience what you put in your mouth. But you wouldn't call yourself a Catholic and then say you don't hold with all that going to church, believing in Jesus and confessing crap. The omega-3 craze has given people the idea that fish is vital for brain health, but, as George Monbiot has pointed out, there aren't enough fish in the depleted seas to sustain this new demand. And this is similar to the claim that people have to eat red meat to obtain iron. You have to ask - given that they eat grass - where do the cows get the iron from?