The playfully patterned wool jumper featuring a single black sheep amid a pattern of white ones headlines Sotheby’s online Fashion Icons sale in September.

The 19-year-old Lady Diana Spencer was photographed in the garment at a polo match in June 1981, soon after her engagement to Prince Charles, who was then heir to the British throne.

Diana was already becoming a style icon —- and one of the world’s most photographed women — and the moment gave a big boost to designers Sally Muir and Joanna Osborne, who made the sweater for their small knitwear label Warm & Wonderful.

A few weeks later, Muir and Osborne received an official letter from Buckingham Palace explaining that Diana had damaged the sweater and asking whether it could be repaired or replaced. The original was returned, showing damage to a sleeve the designers thought might have been caused by snagging on Diana’s diamond and sapphire engagement ring.

A new sweater was knitted and dispatched, and Diana was photographed wearing it with white jeans and a black ribbon tie at an event in 1983.

Some have speculated Diana liked the “black sheep” pattern because she felt like an outsider in the royal family. Emma Corrin sported a replica as the young Diana in season four of “The Crown.”

Warm & Wonderful still makes a cotton version of the design, which sells for 190 pounds ($250). Osborne discovered the original lying forgotten in a box in an attic earlier this year.

Cynthia Houlton, Sotheby’s global head of fashion and accessories, said “this exceptional garment, meticulously preserved, carries the whispers of Princess Diana’s grace, charm, and her keen eye for fashion.”

The sweater, which has an estimated price of $50,000 to $80,000, will be on display at Sotheby’s New York showroom from Sept. 7-13. Online bidding opens Aug. 31 and runs to Sept. 14.

The record for a piece of Diana memorabilia is $604,800 for a Victor Edelstein-designed ballgown sold by Sotheby’s in January.