The pictures, which were discovered at the bottom of a trunk in an attic, are among thousands of precious items on sale at the Antique and Fine Art Fair in the Harrogate International Centre.

The Christmas Dinner, which shows three mice eating a plum pudding, is signed: "With best wishes for Christmas from Beatrix Potter, December '93."

The other watercolour, Rabbits Playing in the Snow With a Sledge is signed "HBP '94".

The pictures were sent to Elinor and Bessie Lupton, who were brought up by their father and governess, Miss Corfield, who encouraged the sisters to write to Beatrix Potter after the death of their mother.

Potter's grandmother was the aunt to the girls' mother. As adults they were part of an influential family who owned five mills in Leeds.

Their father helped to found Leeds University and Elinor became Lady Mayoress of Leeds.

The cards were discovered in a brown envelope at the bottom of a trunk in an attic when a member of the family started to go through its contents.

The Christmas Dinner, which is priced at £19,000 and Rabbits Playing in the Snow With a Sledge, priced at £15,000, are being sold at the Harrogate Antique and Fine Art Fair by Kenneth Hurley, of Llanover in Monmouthshire.

He said: "It is very rarely that an unknown Beatrix Potter watercolour comes to light and these two fresh, strongly coloured and finely executed paintings must be quite unique in that they have spent more than a century forgotten in a trunk."

The Antique and Fine Art Fair in the Harrogate International Centre is open Thursday 11am-8pm; Friday 11am-9pm; Saturday and Sunday 11am-6pm; and on Monday 11am-5pm.