D-day for Damien: Is Hirst about to turn the art market on its head or finally come a cropper?

On 15 September Damien Hirst has a surprise in store for the art world and perhaps also for himself. At the New Bond Street Sotheby's he will mount an audacious two-day auction, entitled Beautiful Inside My Head Forever, of 223 artworks, made by himself and to be sold by himself. This is a money-spinning feat by a living artist on a scale never attempted by an auction house or an art gallery. For this reason alone there are mutters along Bond Street that Hirst's sale will be the moment that his bubble bursts.

Damien Hirst is the richest artist in British history. He lives high off the hog in 300 rooms at the violently Gothic Toddington Manor in Gloucestershire, where he plans to work less and, eventually, show off his stuff to the world, like a stately home owner – "I'm no longer an enfant terrible, I'm an old-age pensioner," he said this month.

If his Sotheby's gamble succeeds, he will add £65m-plus to his pile, just like that. The bid looks dangerously greedy but, according to Hirst's people, it isn't at all. The Sunday Times Rich List credits him with more than £100m, which Hirst confirmed 18 months ago was about right; others think his value is more in the region of £500m and the outcome at Sotheby's is peanuts to him. He is so wealthy, claim his friends, that he has spent £100m on art in the past nine months alone. His spree began with a £16m Francis Bacon self-portrait at Sotheby's New York last November, which joins three others at Toddington. Opinion is that his September sale will pay off any outstanding debts to the auction house.

Sotheby's says Hirst decided on a grand auction two years ago, following his £11m mass sale of art in 2004 from Pharmacy, his Notting Hill restaurant. "Auction is a very democratic way to sell," Sotheby's quotes Hirst. "It feels like a natural evolution for contemporary art. Although there's risk involved, I embrace the challenge of selling my work in this way."

The challenge, though, is not for Hirst but for his dealers. Jay Jopling at White Cube, Harry Blain at Haunch of Venison and Larry Gagosian of New York have charged him commissions from 10 per cent to who knows what for two decades. Prices for Hirst now reach £9.6m at auction – that record at Sotheby's in June 2007 for the medicine cabinet Lullaby Spring made him, briefly, the world's most expensive living artist – and prices for his art spiral yet higher in private, so the withdrawal of Hirst's business will be felt acutely by Jopling et al.

It might be argued, of course, that Hirst is merely taking back for himself the rewards he has fed them for decades. Yet, cutting out the middleman is a risk. Dealers have one great advantage over the sale room: they can take their time finding a rich collector willing to pay the astronomic prices Hirst now demands. Sotheby's in September, by contrast, will be on-the-spot triumph or disaster. Failure to sell on the day is death. "Unsold at auction" pins a shadow to an artwork that can take years to disappear.

The thinking on Bond Street is that the top pieces from Beautiful Inside My Head Forever will succeed – The Golden Calf, estimated at £8m to £10m, and the other beasts in formaldehyde: a tiger shark (£4m-6m), a unicorn (actually a foal, £2m-3m) and a zebra (£2m-3m). Those Hirst pieces that deal with the themes of death, decay and here today, gone tomorrow are the works museums want, the art by Hirst that is likely to last.

The doubt lies elsewhere – with the mass-produced spot, spin and butterfly pictures manufactured by Science Ltd to which Hirst's main, if not only, contribution is the concept, and which have become exceedingly numerous over the past decade. For the spins, which are merely decorative, Hirst is asking under £100,000; but the spots, some painted on gold, and the butterflies are estimated at up to £1m. These are ambitious sums for works which the artist admitted in 2001 to barely touching – he did exactly five of them, all "shite", and "as soon as I worked out how to do them, I employed other people," he immortally declared. "The best spot painting you can have by me is one painted by Rachel [Howard]."

One of his major backers, the Cork Street dealer Helly Nahmad, has gone public with his disillusion. He announced this year that "he no longer believed in Hirst at these prices" and that he'd sold up and grabbed his profit.

Nahmad was one of the 1990s forcers of the Hirst market, the bidder who broke the six-figure barrier for his work before Charles Saatchi dared. What will make the sale at Sotheby's an unmissable piece of theatre is that the world will get an answer to the question: how many others think like Nahmad? Rubbish, says the Hirst circle to the disaster theory. The best Hirsts are headed for £25m each ; the auction will be fine.

Hirst's output isn't a mountain – one estimate puts it at between 3,000 and 4,000 works; it's tiny compared with Warhol's (19,000-plus artworks) or Picasso's (over 24,000). But the 223 lots at Sotheby's have been carefully calculated. Hirst knows what his buyers in Tokyo, Los Angeles and Hoxton want and he's made it for this auction. No, say his people, he hasn't been starving the market and driving up prices since 2006 by hoarding works for this auction. He's been selling new art quite normally through dealers as well; 223 lots isn't flooding the market and breaking the rules, it's an exact flood to fill an exact hole.

But Hirst himself has heard the warnings. He slipped this casual remark into an interview with Sotheby's television channel this month: "I was going to use this [sale] to end everything, the formaldehyde works, the butterflies, the spots and the spins. Well, the butterflies are going to stop. I'm going to stop the spins. Then with the formaldehydes there are a few works I want to make – a crucified cow is half-finished – while one spot painting is underway that could take 20 years." One interpretation of these remarks is that Hirst is telling us 15-16 September is your last chance to buy new work of his.

My guess is that Hirst will get away with it. Last year, the gods issued him a warning against hubris: he sold the diamond-encrusted skull, "For the Love of God", for £50m, but the buyers were himself and three investors, which is not the same thing as a profit. But Hirst loves excess and without taking risks, he's dead. Art is about life and the art market is about money, he proclaims. "I'm not under any illusions. I don't think I'm the best artist that ever lived." But he'll know, in two weeks, if he's the most expensive alive.

For more information on the sale on 15-16 September at Sotheby's, 34-35 New Bond Street, London W1, visit www.sothebys.com