A vividly-coloured backcloth used at the 1938 Glyndebourne Festival Opera has been discovered languishing in a garage at the East Sussex estate.
The painted cloth backdrop was used as scenery for what was said to have been the first professional British performance of Verdi's Macbeth opera.
Glyndebourne's executive chairman, Gus Christie, found the item - the work of German stage designer Caspar Neher.
It will be added to the country house and concert venue's archive collection.
Mr Christie said: "I was amazed to find this piece of Glyndebourne's history in relatively good condition.
"The backcloth would have been painted by hand and it is remarkable how the colours have remained so vivid.
"The piece gives us a snapshot of what this production must have been like when it was first shown to audiences almost 70 years ago."
A Glyndebourne spokeswoman said the backcloth was 21ft high (6.4m) and 47ft wide (14.3m), and would have been used for the opera's finale "to make the greatest possible use of the massed chorus dressed as an army with pikes and shields".
The 2007 Glyndebourne Festival Opera runs from 19 May to 26 August, with the venue's third new production of Verdi's Macbeth among the performances.