A jumper worn by Britain’s late Princess Diana and depicting a black sheep is headed for auction later this summer with a price estimate of up to $80,000.

Designed by knitwear label Warm & Wonderful, the sweater, which will headline Sotheby's Fashion Icons August 31 to September 14 online sale, was rediscovered in an attic earlier this year by one of the brand's founders.

Diana first wore the red jumper, which depicts a lone black sheep among rows of white sheep, to watch then Prince Charles play in a polo game in June 1981, a month before they were married, sparking speculation over its potential significance.

After it was damaged on the wrist, her private secretary Oliver Everett wrote to Warm & Wonderful co-founder Joanna Osborne asking if it could be repaired and the jumper was sent back. A few months later, Diana received a replacement, which she was photographed wearing in 1983.

Osborne found the original in a box in her attic in March.

"If you’re Princess Diana, certainly you have access to lots of pieces of apparel you could choose to wear," Cynthia Houlton, global head of fashion at Sotheby's, told Reuters at a press preview in London on Monday.

"And the fact that she wanted a replacement and then again two years later wore ... the replacement sweater, I think speaks really volumes from her how much this sweater meant to her.”

The jumper, which is being sold with Everett's two letters to Osborne, has a price estimate of $50,000-$80,000.

Earlier this year, Sotheby's sold a purple, velvet, strapless evening gown worn by Diana, designed by couturier Victor Edelstein for his autumn 1989 collection, for just over $600,000, five times its pre-auction estimate.

The jumper and letters are on display at Sotheby's London until Wednesday. They will go on show in New York in September.