Queen Mary meets Elvis Presley...

They queued discreetly this week at the back entrance to Christie's auction house in the smart streets off Piccadilly, clutching glossy catalogues (price pounds 30, including entrance to the exhibition) and souvenir carrier bags. Women of a certain age, dressed for the occasion in floral prints and pastel suits, they might easily have been on their way to a wedding. Unfazed by the discreet sign warning of at least an hour's wait, they passed the time flicking through the catalogue, exclaiming like proud parents leafing through a family album.

One woman, who had travelled from Hampshire for the day, explained: "To us she's always been a fairy princess. Of course, we wanted to see her dresses." Inside, however, even Princess Diana's most ardent fans were in for a surprise. Her gowns, which are to be auctioned for charity in New York at the end of this month, are being sold "as is" - saleroom code for without any guarantees as to their condition. One of the very first exhibits, listed as a romantic balldress of pale-blue, sequinned tulle by Emanuel, featured an uncatalogued light-brown food stain on the skirt. "What happened there, I wonder?" someone wondered aloud, prompting silent speculation about an embarrassing moment at some long-ago royal dinner. What does a princess do when, just like the rest of us, she has the misfortune to drip soup on her brand-new frock?

Further into the exhibition, other dresses were coming under not entirely favourable scrutiny. One visitor remarked that Lot 53, a sequinned, full- length coat-dress by Catherine Walker, "looked just like a dressing gown". Lot 11, a short, midnight-blue, cocktail dress, also by Catherine Walker, resembled a nurse's uniform from some expensive private clinic. The Princess's red period, vividly represented by a scarlet-and-silver-tartan dance dress by Bruce Oldfield and a short, lace, cocktail dress by Catherine Walker, prompted a design lecturer from north London to step back and exclaim: "It's like death on the roads!"

A smartly dressed, middle-aged woman, who had made a special journey into town with her mother to see the dresses and have lunch at nearby Fortnum's, loyally observed that most of the outfits looked much better in photographs, as though the Princess somehow brought them to life when she put them on. "They look very different up close," agreed a 27-year- old graduate student from Los Angeles. She had heard about the sale on both British and American television and had come to Christie's out of curiosity, not an urge to buy. "I'm not sure I'd want to be known as the woman who wore one of Princess Diana's dresses," she admitted.

In any case, the guide price of $5,000 per dress (plus the buyer's premium of 15 per cent and New York sales tax) places them well beyond the average student's budget. The proceeds of the sale, which consists of 80 dresses, are to go to half a dozen charities, including the AIDS Crisis Trust and the Royal Marsden Hospital Cancer Fund. If, however, the pattern established in the past couple of years by other celebrity sales - the contents of Jackie Kennedy Onassis's 5th Avenue apartment, or the effects of the late US ambassador to France, Pamela Harriman - is repeated, the charities should receive far in excess of the projected $400,000.

What the new owners will do with their purchases is another matter. The dresses cannot be tried on in advance and the catalogue does not give details of sizes, although the sale is a testament to the sharp fluctuations in the Princess's weight during her royal career. In that sense, the preview was not an entirely comfortable experience, the occasional pulled thread or faint stain drawing spectators into an unexpected intimacy with their absent owner - a woman who, unlike Kennedy or Harriman, is still very much alive. It felt, remarked one visitor, a bit like rifling through someone's underwear drawer.

The Princess is also, if this peep into her wardrobe is anything to go by, recovering from a prolonged state of confusion about her identity. Although dates are not provided for every lot, the dresses fall into several discrete categories, beginning with the "fairy princess" look which followed her marriage to Prince Charles in 1981. The Emanuel balldress which opened this week's exhibition is typical of the Princess's taste at this time. Its floaty skirt and abundant frills suggest it was made for a little girl who had not yet got over her urge to dress up. In sharp contrast are the severely tailored evening gowns in muted colours, heavily encrusted with beads and pearls, which hark back to an earlier era, like accidental inclusions from a costume museum.

Most revealing of all is the Princess's preference for swathes of fabric, pleated and ruched over the bust and sometimes the hips in a way that recalls the old Chinese practice of binding the body after pregnancy. Classic examples of this style are a Catherine Walker balldress in burgundy velvet and a white chiffon gown, overprinted with purple tulips, made by the same designer in 1989. No matter how revealing the shape, the effect is to suggest that the Princess is wearing soft body armour - a reflection, perhaps, of her discomfort with her public role as she came to feel increasingly unsupported by her husband.

Glamour and tragedy are a potent combination. The Princess's taste in clothes may be inconsistent and eccentric - the overall impression of the preview is Queen Mary meets Elvis Presley in his rhinestone cowboy period - but the sale has little to do with fashion. The lucky few who make it into Christie's Park Avenue saleroom at the end of this month, chosen by lottery and required to produce bank references, will be the latest initiates in a recent but powerful cult: relic-hunters at the shrine of St Diana.