The whisky group said master distiller Ian MacMillan discovered the casks in the distillery ledgers at the Islay-based Bunnahabhain distillery.
Unpeated - as is the Bunnahabhain style - unchill-filtered and carrying an abv of 41.7%, the single malt is priced at £1,999 per bottle.
MacMillan said: “It was an exhilarating moment to stumble on such a special find. There are very few 40 year old Islays and unlike any other, Bunnahabhain’s taste is considered unique because the distillery does not heavily peat the fine malted barley.”
Only 750 bottles of the Islay single malt are available to worldwide markets.
Michelle Lansdowne, senior brand manager at Burn Stewart Distilleries, said: “ We are expecting a good deal of market interest in the 40 YO and with such limited availability of this one-off prestige bottling, it is sure to appeal to international collectors of rare malts.”
The so-called ‘Turney casks’ were filled by Glasgow wine merchants J G Turney over 40 years ago.
Burn Stewart Distillers - which also owns single malts Tobermory in Mull and Deanston in Doune, near Stirling - exports Bunnahabhain to over 30 countries globally.