BIG breasts, it has emerged this month, have become posh. First out of the dress were two belonging to actress Elizabeth Hurley - husky Home Counties girl and paramour of floppy-haired heart-throb Hugh Grant. Both were on display at the premiere of Grant's film Four Weddings and a Funeral, artfully decorated by Gianni Versace and a dozen over-sized safety pins.

This week, those on view at a celebrity film screening belonged to the super-posh Serena, wife of Viscount Linley, and Susannah Constantine, Lord Linley's former girlfriend.

The emergence of the upper-class bosom is of great sociological significance since, in recent times, big breasts have been thoroughly plebeian. Tacky, white-stilettoed Essex girls had big breasts. Air-headed, silicone-enhanced Hollywood starlets had big breasts. Cockney darlin' Barbara Windsor had big breasts, always the excuse for a leery joke from Sid James. The men who liked big breasts were common too: they liked Benny Hill, read the Sun and went on cheap holidays to Blackpool, where you can still buy saucy seaside postcards saying, 'No, no, Miss - I said it's a contest of WITS]'. A-cups equalled A-levels; C-cups equalled CSEs.

The first usage of the word 'cleavage' to describe breasts was recorded in 1934. Dame Anna Neagle recalled in her autobiography that the Purity League of America banned her film Nell Gwyn, after being offended by her costume. 'My dress revealed too much 'cleavage'. It was the first time the word was used in this particular context. 'What is 'cleavage'?' Herbert (director Herbert Wilcox, who became her husband) asked the Purity League. 'Well, it's a shadow between two mountains,' was the reply.' Since then the message has been fairly consistent: breasts are down-market and dirty.

But now we have the big posh breast. Like so much in upper-class life, these assets are well-protected - cushioned in expensive, flattering designer dresses. With this protection, there is no danger of the posh breast falling out: it is cooly offered, to look at but never to touch. The posh cleavage is curiously unsexual: pale, smooth, serene.

Why now? Partly, I suspect, the upper-classes - so often slow when it comes to popular culture - have finally wised up to the Wonderbra/Ultrabra phenomenon. Everyone else has been wearing them since the beginning of the decade - sales of Wonderbras have soared from 400,000 to 1.7 million in the last two years - but only a horse could have missed the recent billboard campaign, featuring Czech model Eva Herzigova.

Second, the high-fashion shape of the moment is not really the underweight, under-dressed waif but the curvy, glamorous babe (prototype: Cindy Crawford). Breasts are now so compulsory that even uncompromising anti-sex symbol Jodie Foster appeared on a recent cover of Vanity Fair in a slutty, satin slip. (Inevitably, she looked like a teenager in her mother's dress.) The only exception to this rule seems to be the mature woman who hasn't had plastic surgery, such as Helen Mirren, pilloried last week by the Daily Mail for having the timerity to wear a vampy, see-through dress at the age of 47.

Ladies keen on exposing their decolletage should take care, however, for some men still mistake posh cleavages for common breasts. Witness the recent obsession the Sun developed with Elizabeth Hurley, who had to issue a writ to prevent the paper turning her into a Page Three girl. Rather that than the other way round, perhaps: a world of sexless Serena cleavages would be very dull.

(Photographs omitted)