Coming in at a cost of over €7 million – the British Guiana 1c Magenta stamp is thought to be, gram for gram, the most expensive item in the world.
First entering circulation in 1856, it’s the only one of its kind to survive from the former British Colony. It measures only 29 x 26mm.
David Goldthorpe, head of books, Sotheby's auction house, tells its story.
"It was discovered by a schoolboy stamp collector, he was Scottish, but he was resident in British Guiana."
"He found it among his uncle's papers, and he recognised it obviously as a stamp, and as a fairly unusual one, but he didn't really take to it, so he swapped it with another local collector for, I think, a group of other foreign stamps."
"And its rarity was gradually appreciated, so that every time it changed hands into other great stamp collections, it fetched more and more money. It's the only major world stamp that's not present in the royal stamp collection."
Shortly after, the emergency substitutes were taken out of circulation – with this being the single surviving stamp.
It made a brief return to Britain in the late 19th century before being bought by an overseas collector.
Previous owners include the French government and the infamous businessman and criminal John Du Pont.
The stamp will be met by an armoured truck when it arrives at London’s Heathrow Airport where it will go on display at the stamp specialist Stanley Gibbons.