The letter, written to four-year-old Marjery Spicer in January 1903, will be auctioned by Bonhams on 11 May. It is being sold by Mrs Spicer's daughter, Rosemary Franklin, who only learnt of its value when she took it to BBC Television's Antiques Roadshow.
In the four-page letter, the children's author told Marjery about her favourite pets, including a white rat called Sammy, some brown rabbits and two squirrels.
She wrote: "I bought two squirrels, but the one called Nutkin - who was much the handsomest - was so very savage I was obliged to take him back to the shop. So I have only one now, called Twinkleberry."
The letter was written six months before the publication of The Tale of Squirrel Nutkin, which became one of Potter's bestselling books. At the time, she was working on illustrations for it.
Sent in reply to a letter from Marjery, whose mother was acquainted with an artist friend of Potter, it is illustrated with five drawings including two of the earliest known images of Nutkin and a picture of Sammy asleep in a four-poster bed.
The handwritten letter, on yellowing notepaper, is accompanied by its original envelope, addressed to Marjery at her home, Colthrop House, Thatcham, Berkshire. Mrs Franklin, who inherited the letter from her mother, was astonished to find out how rare and valuable it was. She took it to the Antiques Roadshow when the programme visited Westonbirt, near Tetbury in Gloucestershire.
"I was stunned," she said. "Until then I did not have a clue of its worth and certainly had no idea it contained one of the earliest illustrations of Nutkin. I read all Potter's books as a child and often wondered if the letter was of any value."
Potter, who was born in London in 1866, created her first character, Peter Rabbit, in an illustrated letter in 1893.
After rejections from at least six publishers, she had The Tale of Peter Rabbit privately printed. Its success prompted Frederick Warne and Co to publish further editions as well as her later stories. She died in 1943.Reuse content