A?RARE stamp with a Camelford postmark is expected to fetch £6,000 at auction on September 9.

The Penny Black stamp would be worth even more — £20,000 — if it had not been torn off its envelope.

The stamp was sent from Camelford on June 30, 1840, only a few weeks after being launched as Britain's first adhesive postage stamp.

What makes it so valuable is not the actual stamp but the undated red Camelford cancellation mark which Spink, the London auctioneers describe as "a great rarity."

Somebody forgot to apply the cross and thus made it a very rare find.

The Stanley Gibbons catalogue price for such a stamp is £15,000 but the experts believe it would be more like

£20,000 if it had been left on the envelope.

The Penny Black issue ended within a few months after it was found that the red ink used for cancellation marks could easily be washed off and the stamps re-used. It was replaced by the penny Red.

The catalogue describes the stamp as follows: "Lot 84 Great Britain 1840 One Penny Black Plate II RI, good to large margins, tied on piece dated 1840 (June 30) by fine and complete undat... £5,000 to £6,000."