Dating from 1840, the item is "the earliest securely dated example of the first postage stamp," according to the auction house that will present it at its "Treasures" sale on December 7.

The adhesive stamp, which features a profile of Queen Victoria, is attached to a document dated April 10, 1840, from the archive of British postal service reformer Robert Wallace, a Scottish politician.

The Penny Black, which introduced a flat rate, was used from May 6, 1840. Before that, the recipient paid the postage cost.

"This is the first ever stamp, the precursor to all stamps, and unequivocally the most important piece of philatelic history to exist," Henry House, head of Sotheby's Treasures Sale, said in a statement on Tuesday.

"Though there are many hugely important stamps in collections both public and private around the world, this is the stamp that started the postage system as we know it."

The stamp is one of three Penny Blacks believed to have survived from the very first sheet of printed stamps. The other two are part of the collection at the British Postal Museum.

"The fact that Wallace signed, dated and issued his note ... gives support to the fact that this is the very first example of a postage stamp, which of course every country now uses," owner Alan Holyoake, a businessman and philatelist, told Reuters.

Holyoake came into possession of the Wallace document some 10 years ago. He undertook a three-year research project to determine its authenticity and the stamp has certificates from The Royal Philatelic Society and The British Philatelic Association.

It is the first of its kind to be offered at auction, Sotheby's said, giving it a price estimate of STG4 million to STG6 million ($7.35 million to $A11 million)

Australian Associated Press