A one cent Magenta postage stamp printed in British Guiana in 1856 is expected to fetch a record auction price of $US10 - $US20 million, Sotheby's said on Monday.
The stamp is being sold by the estate of the late John du Pont, who died aged 72 in a Pennsylvania prison in 2010 where he was serving a sentence for the shooting of a US Olympic wrestler in 1996.
The stamp is the only one of its kind known to exist, and will be offered at auction in New York on 17 June.
"It is one inch by one and a quarter inches, said Sotheby's vice chairman David Redden. "Our estimate on this stamp is $10 - $20 million.
''That seems like an awful lot, but in the great scheme of things, across the entire collecting world, the most extraordinary objects in every field, that price suddenly becomes a little modest."
The one-cent Magenta was printed in British Guiana in 1856 after a shipment of stamps from England was delayed
Royal Philatelic Society London expert committee chairman Chris Harman said the stamp was without peer.
"It's one of the first stamps in the world, 1856, British Guiana was one of the first countries in the world to issue their stamps, and this was a locally printed stamp, of which there are very few four-cent, and there's only one one-cent," he said.
It has not been on public view since 1986, when it was exhibited at a stamp show in Chicago.
The current auction record for a single stamp is 2,875,000 Swiss francs (approximately $US2.2 million), set by the Treskilling Yellow in 1996.