One of the Penny Black stamps is valued at £15,000
A stack of Victorian letters hidden in the attic of a derelict building has proved a valuable find after a rare Penny Black stamp was found.
It is among 27 Penny Blacks, first issued in the UK in 1840, and found on letters impaled on spikes in the loft.
It has a catalogue value of £15,000 but its condition means auctioneers in Denbighshire value it at £5-6,000.
The letters were encrusted with bird droppings which may have helped preserve the stamp collection of 400.
The stamps, including Penny Reds, were found in the property in north Wales, with many valued at £200 each.
Colin Knights, owner of Aqueduct Auctions in Froncysyllte in Denbighshire, said they were found by chance as the building was being cleared.
He said the owner of the stamps had almost emptied the letters into a tip, "but by chance happened to see what was underneath a solidified crust of bird droppings".
After removing the crust, old letters, some complete with stamps, were revealed.
Among the find were 27 Penny Black stamps and 400 Penny Reds and Penny Red-Browns issued between 1841 and 1848.
The most valuable Penny Black is from a rare issue, plate 11.
Mr Knights said: "It seems an old business had saved all their old letters by impaling them on letter spikes, and they had been kept in the roof space where birds had been getting in."
Most of the envelopes were postmarked for nearby areas including Wrexham, Mold and Liverpool, a few were marked to London and Manchester.
Mr Knights added: "It's interesting to think what other hidden items are lying about and awaiting discovery.
"We just don't know what interesting and valuable items may still get thrown away because no-one looks at them properly."
According to Mr Knights there were many more of these old letters but someone had thrown half of them away before the roof space was cleared.
The auction will be held at Aqueduct Auctions on 15 May.
"It's difficult to say how much the stamps will go for," he said.