Blake was paid 24 guineas for his illustrations

The pictures, which were only discovered in a second-hand book store in 2001, were considered an exciting find by scholars of the poet and artist.

But the set of 19 illustrations to Robert Blair's Gothic poem The Grave were sold to a foreign buyer for �5m earlier this year.

Libby Howie, the London art dealer who brokered the sale, said that the anonymous buyer is still considering what to do with his acquisition.

She said her client, a collector of 19th and 20th Century paintings and drawings, was aware that taking them out of the country would be a controversial move but would not rule rule it out.

Academics had not realised that seven of the 19 pictures even existed until they turned up in a bookshop in Glasgow.

"They are not the kind of objects you can forget about, they are the kind of things that scholars want to examine," said Ms Howe.

"What he has bought is a little bit of Blake scholarship and they are certainly not just going to go into a bank vault.

"But no decision has been made about what to do with them yet."

'Upsetting'

The pictures were painted in 1804 to illustrate poet Robert Blair's The Grave

David Scrase, keeper of paintings at the Fitzwilliam Museum in Cambridge, which houses a large collection of Blake's paintings and etchings, said it would be "disappointing" if the watercolours left the country.

"They are very beautiful and in very good condition, which is very exciting," he said.

"If anything of this kind leaves the country, it is always upsetting."

The appearance of the illustrations caused a legal wrangle between the bookshop and two art dealers who discovered them.

But a settlement was reached that meant that the proceeds of the sale were split between the parties.

The works, all original drawings, were commissioned by engraver Robert Cromek to illustrate a new edition of The Grave in 1804.

Until the watercolours were found more than a year ago, there had been no trace of them since Cromek's widow sold the originals at an Edinburgh auction in 1836.

The only known record of them had been a set of engravings of 12 of the drawings, produced by Luigi Schiavonetti.

The �5m sale dwarfed the previous record of �195,000 paid for a Blake work at Sotheby's in 1998.