The painting is expected to fetch up to �12,000

The work by 18th Century artist Thomas Girtin, is expected to fetch up to �12,000 at Sotheby's.

The watercolour - measuring 10.5cm by 16.5cm - is inscribed on the backboard "On the coast of Northumberland. T.Girtin".

Girtin was a friend of renowned British artist, Joseph Mallord William Turner.

Much of London-born Girtin's life remains a mystery. He died at the age of 27 from an unknown illness.

However he is regarded by many as one of the best painters of his generation.

Dock gates

A spokesman for Sotheby's said: "This important and unrecorded watercolour by Girtin depicts the town of Seaton Sluice.

"The town, as shown, consists of a handful of cottages positioned on a small tidal basin with two outlets, one natural and one man-made.

"The man-made channel can be seen in the foreground of the picture, with its ingenious system of dock gates.

"In the distance, tall masts of ships appear above the land, anchored near the natural channel of the Seaton or Holywell Burn.

"The present watercolour dates from 1796, the year when Girtin came of age and set out on his first independent tour, travelling to Yorkshire, Northumberland and the Scottish Borders.

"He continued travelling for the rest of his career visiting the West Country and perhaps Yorkshire again in 1797.

The Laing Art Gallery in Newcastle has three watercolours by Girtin.

However, a spokeswoman said the gallery was unlikely to make a bid for the painting.

The auction takes place at Sotheby's in London on 26 March.