The last time that a comparable drawing by the Renaissance genius appeared on the market was 1936 and Christie's is hesitant about suggesting an estimate for The Holy Family with the Infant Baptist, a tender study in chalk and ink. Its only gauge is a Raphael, a man's head and hand, sold in the 1984 Chatsworth sale for pounds 3.5m.
The Michelangelo, unusually large in size and complex in technique, has been dated to the early 1530s, when the artist was working in the Medici chapel in Florence.
Noel Annesley, deputy chairman of Christie's, said: 'This is not just a drawing by Michelangelo, but a major drawing by Michelangelo.'
He did not foresee problems over exporting the work, as it had already been offered to the British Museum and other public collections: 'They would have loved to have bought it. They couldn't afford it.' As some two fifths of Michelangelo's surviving drawings are in British collections, its export would be 'sad, but not a disaster'.
Professor Michael Hirst, a leading Michelangelo scholar, said that the number of drawings in private collections was less than a handful. 'It would be wonderful if this one was added to the collections of the BM or the Ashmolean . . . Maybe some philanthropist will step forward and find the money.'
The drawing can be traced back to one of Michelangelo's descendants (from whom Napoleon's commissioner in Italy bought it) and on to Sir Thomas Lawrence, the English portrait painter who built up a superlative drawings collection.
All the Michelangelo drawings at the Ashmolean, Oxford, and 48 of 85 at the British Museum were owned by Lawrence. The seller of Christie's example is an anonymous British collector.
Christie's also unveiled yesterday an important Holbein miniature - two jewel-like portraits within a single frame - of Thomas Cromwell, Earl of Essex, the most powerful figure at the Court of Henry VIII. The estimate is more than pounds 500,000.
A set of 24 photographs by Alexander Rodchenko, the Russian photographer, was sold by Christie's yesterday for pounds 198,375. The highest price was for Steps, 1935, which sold for pounds 32,200 (estimate pounds 15,000-pounds 20,000).
A set of Rodchenko prints sold for pounds 495,165, less auctioneer's commission, at Christie's last October, not pounds 892,320 as reported in the Independent yesterday.