Picasso, Cézanne, Magritte, Bacon – London prepares for £500m art auctions, Yazzy's at www.williamverdult.com

By Art investors from around the world are expected to descend on the capital as rarely seen masterpieces, including works by Francis Bacon, Andy Warhol and Picasso, go under the hammer.

This month's sales of Impressionist, Modern and contemporary art works will be closely watched by experts, who are wondering how long prices can continue to rise following the sub-prime mortgage crisis and talk of a looming US recession.

Charles Dupplin, the head of the Fine Art and Private Client division of Hiscox, the largest insurer of fine art in Europe, said that this month's sales could be a good indicator of things to come.

"The main sales in New York in 2007 were very strong but that was just before the credit crunch. It will be interesting to see whether the actual depth of the market is still there now in terms of the number of bidders and the percentage of lots that sell.

"There are some exceptionally lovely things on sale in London this month. It will be interesting to see whether people's love or art ... outweighs the general negativity bidders might be feeling."

Today, Christie's Impressionist and Modern Art sale will put eight works by Egon Schiele under the hammer, as well as 35 pieces by Surrealists such as René Magritte and Marc Chagall.

Tomorrow, works by Picasso, Jawlensky, Sisley, Monet, Cézanne and Renoir will be auctioned at Sotheby's. Among the most valuable works on the list is a portrait by Picasso of Dora Maar, the photographer and painter who became his lover in the late 1930s. The work is expected to fetch between £6.5m and £8.5m, while the most important triptych by Bacon ever to appear on the open market is expected to set a record when it leads Christie's post-war and contemporary art sale on Wednesday. It could fetch £25m.

The piece, Triptych 1974-77, is the fourth and last in the Black Triptychs series painted by Bacon in response to the death of his lover, George Dyer, who committed suicide in their hotel room on the eve of the artist's retrospective at Paris's Grand Palais in 1971.

None of the other triptychs by the artist, who died in 1992, has ever appeared on the open market and only one is in private hands.

Later this month, a 1969 work by Bacon, Study of a Nude with Figure in a Mirror, is estimated to sell for between £18m and £25m at Sotheby's. The auction house has guaranteed it will sell for £18m – a record for a London auction.

"These sales will be a bellwether," said Melanie Gerlis, art market editor at The Art Newspaper. "Some people argue that art is like gold – it's a safer investment than property or shares because it doesn't fluctuate in the same way. But it looks to me that there are lots of people trying to sell things at the moment. Maybe people want to sell before the market falls. My view is that it's probably only a matter of time before art prices are affected."