Alice Martin, Mount Stuart's head of historic collections, believes it was bought by the third Marquess of Bute, an antiquarian and collector, who died in 1900.The Descriptive Catalogue will need updating.
The trust which runs the Gothic revival house had been researching the collection of books, paintings and historic items and called in experts from Oxford University to assess the authenticity of what had been claimed as a First Folio.
Apart from its cultural value, verification makes the book extremely valuable. A copy owned by Oriel College, Oxford sold for about £3.5m in 2003.
Authenticating a copy involves a series of technical checks on, among other things, the watermarked paper and printing process.
Books A new (as in newly discovered) First Folio has been found in Bute. The BBC has the full story: