A record for the world's most expensive postage stamp is likely to be set this weekend, when a Swedish stamp, described as "The Mona Lisa" of the philatelic world, goes on sale with an estimate sale price of around €2m.

The circa 1855 Swedish "Treskilling" stamp - which weighs less than a gram - will go under the hammer on May 22 at Feldman Galleries, Geneva, without a reserve price.