IT is one of the rarest and most valuable books in the world, viewed by collectors as being akin to the Holy Grail.

And now the number of Shakespeare First Folios known to exist has risen by one, after a copy was found languishing on the bookshelves of Mount Stuart House on Bute.

The First Folios are the original collection of 36 of the bard's plays, published in 1623 shortly after his death by two of his contemporaries.

Only 750 were ever produced, and just 233 were known to have survived before the latest version was unearthed in Scotland.

Among their pages are the original texts for plays such as McBeth, Julius Caesar, The Tempest and Twelfth Night and others, and it is only because the Folio was published that they have come to audiences down through the ages.

Only one other copy is believed to reside in Scotland among the collection at Glasgow University, and last one to be sold at auction fetched a price of £2.5 million.

Alice Martin, Head of Historical Collections at Mount Stuart House, said that the find was "hugely exciting" and that it had been kept under wraps until it could be fully verified.

The First Folio was listed among the stately home's collection, but had never been authenticated and few believed it was an original copy.

Ms Martin said: "I've been working here about a year and I kept walking past it on the shelf, but I didn't think for a minute it could be a first edition.

"Then one day I got it down and started going through it, and I began to think 'wait a minute, this might be real after all'."

The Folio, which is split into three volumes and bound in goatskin, was passed on to Emma Smith, Professor of Shakespeare Studies at Oxford University, who undertook painstaking research and was able to authenticate it as a first edition.

It is believed that the volumes were the working copy of Isaac Reed, a well-connected literary editor working in London in the 1700s, which became part of the Collection at Mount Stuart House in the 19th century.

When the Folios were first published, they were sold as loose pages and it was up to the owners to have them bound, meaning that few are alike.

Professor Smith said: "This is an exciting discovery because we didn’t know it existed and it was owned by someone who edited Shakespeare in the 18th Century.

"It is an unusual Folio because it is bound in three volumes and has lots of spare blank pages which would have been used for illustrations.’

It is hoped that the The Folio will be the first of many significant discoveries in the Bute Collection at Mount Stuart, which is being catalogued. The collection was put together over 600 years and includes landmark works of British portraiture from the 18th and 19th centuries, Italian masterpieces from the 16th century, and Dutch and Flemish Old Masters.

Ms Martin added: "In terms of literary discoveries, they do not come much bigger than a new First Folio, and we are really excited that this has happened on Bute."

"But it is just the tip of the iceberg for the undiscovered material in the remarkable Bute Collection, and we are working with scholars from universities including Glasgow, Dundee, Stirling and Oxford to share our collections with the Scottish public."

The Folio is will go on display at Mount Stuart until 30 October. Although other First Folios are also being shown in other locations this year, the Mount Stuart version is the only one which has never been seen by the public before.