The watercolour windfall worth £5m

An export licence is to be sought in the next few months, and it is believed that the rare portfolio, by one of Britain’s foremost Romantic artists, will become part of an overseas collection.

"Some of the paintings represent the quintessential Blake," said Libby Howie, the London dealer who arranged the sale.

"The drawings are absolutely fully rendered. I have a profound belief in their importance and beauty."

The works are believed to have come onto the market after a house clearance, following the death in Glasgow of their former owner, who never knew their worth. In 2000, they passed into the possession of Caledonia Books, on Great Western Road.

Three years on, the move to export follows a bitter legal wrangle between the Glasgow bookshop and two Yorkshire booksellers who acquired the Blake paintings and helped to ascertain their real importance.

An out-of-court settlement was agreed recently between these parties, with a strict confidentiality agreement that prevents the dealers talking to the media.

However, it is clear that works once believed to be worth just hundreds of pounds have offered up a huge reward for all of the traders involved.

The forthcoming export of the works appears to be the final twist in an extraordinary tale of lost treasures, good luck and, inevitably, a degree of frustration for some of the players who failed to realise the true worth of the objects in their possession.

Painted in 1804, Blake produced the 19 watercolours to illustrate a new edition of The Grave, by the 18th-century Scottish poet, Robert Blair, to be published by the painter’s friend, Robert Cromek.

However, Cromek decided Blake’s work was unsuitable for the edition and hired a fashionable engraver, Louis Schiavonetti, to produce the final images for publication.

In the event, only 12 of the artist’s highly-finished watercolours were copied for the publication, and seven of the works discovered in the portfolio were unknown to modern art historians when the portfolio finally resurfaced.

On Cromek’s death in 1812, the works passed to his widow and then through another collector before they were auctioned in Edinburgh in 1836, fetching just over 1.

The subsequent history of the paintings had been unknown until the appearance of the current edition of the specialist magazine, The Art Newspaper. This uncovered the fact that, after the 1836 auction, the Blake works passed down through a dynasty of Bedfordshire watercolour artists: John Stannard (1794-1882); his son Henry John Stannard (1840-1920); his grandson Henry John Sylvester Stannard (1870-1951); his great-grandson in turn, and then to a nephew in Glasgow.

Throughout this period, the works were stored in a leather portfolio, and remained intact. When the portfolio passed to Caledonia Books, run by Maureen Smillie, neither the Stannard family nor Caledonia Books had any idea of the real value of the works it contained.

Ms Howie was sympathetic to the family and the booksellers.

"I can understand how people could see them completely differently, and don’t imagine they are valuable," she said.

"In a family, I can see how over many generations, the knowledge of what these things are gradually disappears. As things are passed down, people don’t leave careful inventories - things just get lost, even in the greatest libraries or galleries."

She added there was a question of context. "If you were offered a great Italian painting in a bric-a-brac shop on Portobello Road, you wouldn’t assume that’s what it was - you would assume that’s what it wasn’t."

It appears, however, that when Dr Paul Williams, an Ilkley-based bookseller, appeared at the Glasgow store, he had an inkling of their potential.

Dr Williams acquired the paintings and contacted a colleague, Jeffrey Bates. The two men decided to take the works for independent valuation.

The valuer, Nathan Winter, immediately realised the importance of the works and called in two experts from the Tate Gallery to verify the find.

One of the specialists, Martin Butlin, concluded the pictures were "arguably the most important discovery since Blake started to be appreciated in the second half of the 19th century".

While the paintings were temporarily quartered at the Tate, the legal dispute between the booksellers raged.

Then, almost immediately, a settlement was agreed. Ms Howie announced the sale of all the works, for a figure well in excess of the 4.9 million contemplated by the Tate.

Yesterday, she confirmed it was "probable" the paintings would leave the UK, though there remained a possibility of a public show.

However, Ms Howie, a Blake enthusiast, had no doubt of the paintings worth, and delighted in their re-emergence.

"What’s wonderful is the drawings are fully rendered drawings. Schiavonetti, in as far as he could, copied them exactly. But the spirit and character of Blake disappeared in the engraving, and now we have it back. It’s wonderful."