Ronald Lauder’s Neue Galerie will be among the sellers at this week’s London auctions — the biggest ever, with a top estimate of $809 million.

Sotheby’s opened an extra room on Bond Street to help accommodate 650 people tonight, and it pitched a marquee in Hanover Square for a day sale. Christie’s impressionist and modern sale has 130 lots; a decade ago, it was 40 or 50. Last winter’s auctions raised $508.7 million, including commissions.

“That speaks volumes about the expansion of the art market,” the private dealer Christopher Eykyn, who is planning to attend the sales, said.

New York’s Neue Galerie, which also is reshuffling its holdings, will offer Egon Schiele’s “Prozession” at a top estimate of $13.7 million and two other Schieles at Christie’s International, after spending $173 million last year on works by Gustav Klimt and Ernst Ludwig Kirchner.

“The only time I sell art is when I want to buy something else,” Estee Lauder International’s chairman and the Neue Galerie’s founder, Mr. Lauder, said in a January 18 interview in New York. “I believe prices will go down but not for the great works.”

The Neue Galerie is selling the Schieles on behalf of Serge Sabarsky’s estate, which forms part of the gallery’s collection, Mr. Lauder said.

While a tide of bankers’ bonuses and emerging-market wealth is buoying asset values, Mr. Lauder said a financial crash might hurt some of the art market’s big spenders one day.

“In the 1980s, we had the Japanese buyers,” he said. “Now, we have the hedge funds and the Russians.”

Twentieth-century art prices jumped 61% last year and have tripled since 1996, according to Art Market Research’s index of expensive works by artists including Willem de Kooning, Mark Rothko, and Andrew Warhol. The index dropped by almost half in five years after the market fell in 1990.

From the former advertising executive Charles Saatchi’s collection, Sotheby’s has Peter Doig’s “White Canoe,” with a $2.35 million high estimate, Marlene Dumas’s “Passion” and a Tim Noble and Sue Webster work. The founder of the British advertising agency M&C Saatchi Plc has been buying American and Chinese works, including a family trio by Zhang Xiaogang, which sold for a hammer price of $1.3 million in October.

The top lots in London, the world’s second-largest art market after New York, are Christie’s $23.6 million Francis Bacon picture of Pope Innocent X, which would lift Bacon into a select group of postwar artists whose works are priced at that level, and Sotheby’s $15.7 million Pierre-Auguste Renoir, from an heir of Revlon founder Charles Lachman.

“All of us are asking how long the boom can last, and it’s in the mind of sellers too,” Christie’s European president, Jussi Pylkkanen, said.

The art may not all hit its top estimates. “The Picassos do not blow my socks off,” London dealer James Roundell, a frequent bidder for Picassos, said.

Renoir’s 1889 “Les Deux Soeurs,” with two chignoned sisters nestled over a book, may be a test of current taste, said Philip Hook, a senior director of impressionist and modern art at Sotheby’s, which valued its evening sale at between $156 million and $220 million.

Schiele’s 1911 “Prozession” is a dark-hued picture resembling a funeral procession with a skullfaced figure and a barefoot woman. It may look less appealing on the wall than the Austrian’s wilted sunflowers, which fetched $20.6 million at Christie’s in June, some art experts said.

“The Schiele looks a tougher prospect [than the Renoir] despite recent big prices in this area,” Mr. Roundell said, referring to the record prices paid last year for Austria’s Klimt. “Prozession” is part of Christie’s February 6 evening sale, which is valued at between $147 million and $207 million.

Mr. Roundell is a fan of the Renoir. “It’s a pretty classic image and not the type of thing that ever goes out of fashion.”

Dorothy Cherry, widow of the Humana Inc. executive Wendell Cherry, will offer Chaim Soutine’s portrait of a red-scarfed man at a top estimate of 9.8 million. Sotheby’s won’t be the first to auction the picture. The Cherrys bought it at Christie’s in 1997 for $2.9 million, according to Sotheby’s and sale tracker Artnet AG.