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Weighing just 0.03 grams, the tiny Treskilling Yellow is thought to be the most valuable thing in existence by weight and volume.
It only survives today thanks to a 14-year-old Swedish schoolboy who rescued it from his grandmother's rubbish bin in 1885 and sold it onto a dealer for the lowly price of seven Kroner.
Now the stamp, which is being shown at London's Festival of Stamps until Saturday, will be auctioned off in Geneva next week.
The three-shilling stamp was first issued in Sweden in 1855 and used in 1857 to mail a letter.
It was printed in yellow by mistake, and should in fact have been green, which has served to make it extremely rare.
It was presumed that some other stamps were produced in the same batch, but no other Treskilling Yellow has ever surfaced.
The stamp has changed hands many times and each time has set a record price.
The last sale was in 1996 when it sold for 2,875,000 Swiss Francs, £1.7 million, to collectors who remain anonymous.
In 2001 a Danish auctioneer, Thomas Høiland, told the Virtual Stamp Club os its value.
He said: "There was hot competition when it was sold last time. People collecting stamps like to have rare things, like to have special things. I think maybe it's because it's been known for so many years, it's special."
See World's Largest Postage Stamp Guinness World Record Video
