The world's most expensive postage stamp, a one-of-its-kind issue printed in Sweden in 1857, was sold at auction in Geneva to an international consortium Saturday, auctioneers said.

One of the rarest known stamps, the "Treskilling Yellow" -- or three-shilling stamp -- was issued by mistake during a print run of eight-skilling stamps, which are yellow. The three-skilling was normally green.

A Swedish schoolboy stumbled upon it in 1885 while rummaging for stamps to sell on for pocket money. Since then it has passed through the hands of a series of stamp collectors, as well a German aristocrat and a Belgian tycoon.

The last time it was sold, in 1996, it fetched 2.875 million Swiss francs (€3.6 million, $4.5 million) at auction.

Stamp auctioneers David Feldman did not disclose the identity of the buyers, nor the price paid this time round.

They did say however that the stamp remained the world's most expensive.

"The consortium members bought the stamp after deeming that it was a solid investment in these times of crisis," they added in a statement.