Bridget Riley would avert her eyes and hurry past the Tate shop. She hates the `vulgar plagiarism of art'...

When you've seen the paintings in the "Mondrian: Nature to Abstraction" exhibition at the Tate, buy the T-shirt, headscarf, cigarette lighter, coffee mug and clock in the shop that sells the catalogue, posters and postcards. But you won't be able to buy the Mondrian socks with bars of bright colour all over them. Celia Clear, who stocks the shop, put her foot down when it came to his socks: "You can't have people walking all over Mondrian." What is startling about tat in the Tate is that it all comes from the artists' estates, manufactured under licence. Today, major art exhibitions come with a lot of products carrying the artist's name, legitimately, and it is up to the gallery shop owner to draw the line. Celia Clear refused the Cezanne beachball and baby romper suit but she did buy the fridge magnets.

The commercial pressures are fierce. Galleries and museums need the cash. And people who've queued for hours to see an exhibition want more than a postcard to take home with them. A million from the gallery-shop tills in the grant-hungry Nineties can't be scorned. The Royal Academy realised this as far back as 1990 when they made money from Monet. Crayons and jigsaws, kaleidoscopes and tea towels, all sickeningly patterned in pastel florals in a feeble impression of Impressionism raised nearly pounds 1.5m in their shop.

The predicted turnover at the Tate Gallery shop next year is nearly pounds 3m, plus pounds 500,000 on exhibition catalogues. The books and catalogues that the Tate publishes are the envy of the world. They sell more than the artefacts which only represent 12 per cent of the shop sales figures but is that enough to persuade the Tate to open the world's best art bookshop at Bankside when it opens next century to show British and foreign contemporary art? Shouldn't they commission special pieces by artists to sell in the shop, "like those tap-tappety shoes by Rebecca Horn," reflects Celia Clear. Artists, dead or alive, are big business with souvenir hunters. But is it right to exploit art and artists in this way? This is the dilemma facing gallery shops today...

I'll bet that Bridget Riley, who hung the paintings for "Mondrian: Nature to Abstraction" at the Tate (opening tomorrow) averts her eyes as she hurries past the Tate shop. She hates what she calls the "vulgar plagiarism of art" contained within it, and has had a few litigious skirmishes to protect her own art from being turned into fashion or packaging. In the Seventies, when her black and white Continuum paintings ended up on flares, or when the chain of Richard shops adopted (for want of a word) her painting The Fall with the slogan "Such Clever Clothes", she sued. Andy Warhol would've made a million.

Sometimes artists are only too happy to knock up designs for mass production. The RA commission special pieces from in-house ARAs for what they call their "continuity line" in the shop.

You may be able to dine on artworks reproduced on plates and mugs but artists don't necessarily make good designers; I am the owner of a particularly ugly Joe Tilson mug, and though Sol le Witt designed the packaging for Nina Ricci's beauty products for men, it didn't turn him into a good graphic designer.

When marketing teams move into art production, artistic licence takes on a whole new meaning. A ball-point pen with a shark floating in clear plastic wasn't actually designed by Damien Hirst, but after you've seen his work in "Sensations", an exhibition of contemporary British artists from the Saatchi Gallery opening at the RA on 18 September, you can purchase the pen in the RA shop. Some artists shown in "Sensations" designed special products for the RA shop, in which case they are branded thus. So Mona Hatoum's Arsehole adorns a pin badge, Sam Taylor Wood's signature is on his T-shirt and the Chapman Brothers designed a limited edition of BT phonecards. Those who haven't shown any commercial interest have their works freely interpreted by RA gallery shop buyer Martin Francis, who explains: "I look at the artist's work to see what associations it has in the gift market."

Will Rachel Whiteread make the connection between her 100 Spaces sculpture in resin and the bars of glycerine soap in inflatable clear plastic packaging for sale in the shop? Having seen Damien Hirst's Holiday, consisting of drug bottles in a cabinet, will you buy an artful assortment of brightly coloured pills in a clear frame? They're placebos, not beta blockers, alasn