It's a blockbuster modern art sale with a difference. Among the work up for grabs is a new spot painting by Damien Hirst, an etching by Lucian Freud and another box by Rachel Whiteread.

But The Whitechapel Auction, to be held at Sotheby's on Friday, is looking to raise at least £2.5 million which will go towards the Gallery's expansion.

Sotheby's is waiving its exhibition premium for the show, which forms part of Whitechapel's £10million plan to double its space in October 2008. A who's who of contemporary art is donating works to go under the hammer, including Peter Doig, Nan Goldin, Gillian Wearing, Cindy Sherman, Gary Hume and Wolfgang Tillmans.

Every one of the 61 painters and sculptors who are contributing work has previously been shown at Whitechapel, known as the "artists' gallery". Commanding the most money will be Raffinose Undecaacetate, Hirst's latest spot painting. Rumours are rife in the art world that it will be his last such offering.

Although the asking price is £400,000 to £600,000, it could well go for a seven-figure sum. In contrast to a pair of Hirst fakes, which Sotheby's had to remove from a print auction last week, the work has been supplied by the man himself.

Hirst gave the Standard a cryptic explanation of his offering, saying: " Information, codes, a paint chart, people, language, beads, plates, eyes, pretty, mathematical, logos, art, geometry, molecules, spots, dots, planets, acne, pills, which way up, or down, what size - big or small, why they exist, what the are trying to say, why they look good... I don't know what the f**k they are."

Gallery director Iwona Blazwick said: "Damien is a great fan of the gallery and wanted to make a statement in support."

The works are on show at Sotheby's, New Bond Street.