The only surviving example of a legendary stamp that sold for one cent in 1856 has gone under the hammer for $9.5m (£5.6m), setting a new record for the most expensive stamp sold at auction and the most valuable object by weight and size.

Sotheby's sale of the British Guiana one-cent magenta postage stamp reinforces its reputation as the world's most famous and valuable.

Recently owned by an American millionaire who died four years ago in a prison cell, the stamp was one of an emergency printing of several denominations by the local Official Gazette newspaper in British Guiana in 1856, when storms delayed a shipment from the UK and the postmaster was in danger of entirely running out of stamps.