The world's most famous postage stamp, a tiny piece of British colonial memorabilia from 19th century South America, is going under the hammer in New York for $US10 to $US20 million ($A11-$A22 million).
It was made in 1856 in Guyana, and Sotheby's will sell the magenta stamp on June 17.
Last bought in 1980 by convicted murderer and American multi-millionaire, John du Pont, it has not been seen in public since going on display at a stamp exhibition in Chicago in 1986.
Since 1922 it has three times broken the record for the price of a single stamp sold at auction, and Sotheby's estimated value puts it way ahead of the last known record of $US2.5 million.
"The estimate is very realistic for the rarity we're dealing with," David Redden, Sotheby's director of special projects, told AFP, adding "this stamp has extraordinary fame and charisma".
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The oldest surviving stamp in the world dates back to 1840, but the British Guiana One Cent Magenta is the most celebrated and in remarkable condition given it is more than 150 years old.
Octagonal, printed in black ink and measuring one by 2.5cm by 3.2cm, it is hinged to a paper backing.
Redden said the stamp was the sole-surviving example of a one-cent magenta, missing even from the British royal family's philatelic collection.
It was rediscovered in 1873 by Vernon Vaughan, a 12-year-old Scottish boy living with his family in British Guyana, who sold it in 1878.
It changed hands several times, and most recently was was bought in 1980 by the late du Pont for record of $US935,000.
In a case that shocked the US, du Pont shot dead Dave Schultz, an Olympic gold medal wrestler, in 1996 and died in prison in 2010.
The stamp is being sold by his estate.
