A PREVIOUSLY unknown portrait of Scotland's bard, Robert Burns, by one of the nation's leading painters has been discovered in England and is now on the market for £2 million.

The portrait of Burns, believed to be by Alexander Nasmyth, was found at a provincial auction in England, but has now been authenticated by an expert and its owner would like to sell it to a Scottish collector or museum.

The painting, shown here for the first time, appears to be the fourth version of a portrait painted in the late 18th century by Nasmyth after meeting Burns: other versions hang in the Scottish National Portrait Gallery, Edinburgh, the Kelvingrove Art Gallery, Glasgow, and the National Portrait Gallery, London.

The owner, who wishes to remain anonymous, is hoping a public gallery will want to own the painting, dated from 1800 to 1810 and authenticated by one of the leading scholars of 18th-century Scottish painting, Dr David Mackie of Cambridge University.

Last night, Dr Mackie said he was originally alerted to the painting because the owner thought it might be a work by Sir Henry Raeburn, his area of expertise.

"I have known about this for about two years, and although it is not a Raeburn, I believe it is a Nasmyth. I make that judgment purely based on its style," he said.

"It is in very good condition and it is a really charming little picture."

The painting - the only one of Burns taken from life in private hands - has also been investigated by writer Jerry Brannigan, who will publish a book on Burns in Edinburgh next year.

Mr Brannigan and the owner are calling the picture, which shows the Bard in an oval mount, with trees and a town in the background, the Shaw Burns, as they believe it was once the property of Sir James Shaw, the Lord Mayor of London from 1805, who was originally from humble beginnings in Kilmarnock.

Sir James, after Burns's death in 1796, became a key benefactor to the Burns family, raising funds for them, finding jobs for Burns's relatives in the East India Company and enlisting the help of Sir Walter Scott in aiding Jean Armour, the poet's widow, financially.

When Sir James died, in 1843, he made a bequest to leave his paintings to the London Corporation, but this was not done: it seems likely, Mr Brannigan's research suggests, his collection was sold to raise money to pay off debts incurred in the 1830s.

This painting was sold and made its way, through the years, to the provincial auction where it was bought by the current owner in its original late 18th-century frame, in a "very dirty state", but with "Shaw" inscribed on its reverse.

A label found inside the frame confirms it is from Edinburgh and the "strainer", which holds the canvas, is from the late 18th century, while the dovetailed construction of the frame is the same as in the other Nasmyth pictures.

Dr David Mackie said X-rays of the picture show preparatory work to be in the style of Nasmyth and he is confident it is by the painter.

Mr Brannigan said: "Everything fits, and it is a fascinating story: Shaw went out of his way to help the Burns family and it would have been natural for him to own this painting. It would be reasonable to date the painting as being in the possession of Shaw between 1800 and 1810, during his period as Lord Mayor of London and Chamberlain.

"The Shaw Burns is therefore very likely to be the first copy Nasmyth made of the original."

The publisher William Creech, who published the second edition of Burns' Poems - Chiefly in a Scottish Dialect, commissioned Nasmyth, to paint a portrait of the poet to be used as a frontispiece for the new Edinburgh edition, with the writer sitting for Nasmyth on several occasions.

The painting, dated to 1787 and owned by Creech, has been in the Scottish National Portrait Gallery since 1882, and is the most well-known and widely reproduced image of the poet.

However, the painter, who lived from 1758 to 1840, continued to produce other versions of the image, with the portrait in London dated to 1821-22, and the one in Kelvingrove dated to between 1792 and 1824.

The owner of the painting believes it could fetch much more than £2m on the open market.

He declined to disclose how much he paid for the work, but said: "Sometimes auctions just miss things and in this case, it seems they did not know everything about the painting and how important it was.

"I saw it and thought it was very interesting. The research done by Jerry [Brannigan] is incredible, I am very impressed by it.

"I have had the experience of finding it and the excitement now - personally I would like to see it in a good Scottish home now."

Imogen Gibbon, senior curator at the Scottish National Portrait Gallery, said: "This is a very interesting discovery. I would say often people approach museums and galleries with what they think is a new portrait of Burns, but often they date from the 20th or late 19th century, but this is appears to be an exception."