Princess Diana’s iconic black sheep jumper which has been forgotten in an attic for nearly four decades is about to go under the hammer.

The garment, to be auctioned at Sotheby’s, has a guide price of £40,000 ($AU75, 870) to £70,000 ($AU132, 770).

Lady Diana Spencer first wore the hand-knitted wool jumper in 1981 while watching Prince Charles play polo.

Lady Diana Spencer wearing the ‘black sheep’ wool jumper in 1981. Top pic, Diana wearing the replacement jumper in 1983.

At the time, the designers of the jumper, Sally Muir and Joanna Osborne, were shocked to see her wearing their design on the front page of a newspaper.

A few weeks later, they received a letter from Buckingham Palace explaining that Diana had damaged her jumper and would like a replacement.

The original jumper with a small hole in the cuff. Pic via Sotheby’s.

A new jumper was made and sent off to the Palace and the original, which was returned to the Muir and Osborne’s clothing label Warm & Wonderful with a thank you note from Diana, was packed away and forgotten about.

Actress Emma Corrin (who played Diana), wearing the ‘statement’ jumper which made an appearance in the fourth season of Netflix drama The Crown.

In March this year, Ms Muir told the Daily Mail she had been “rummaging through the attic searching for an old pattern,” when they spotted a small box.

“Inside was Diana’s original red, sheep sweater,” she said.

Bidding for the original jumper will open in August.