The designers of Princess Diana's sheep sweater thought they accidentally gave it away decades ago

• The designers of Princess Diana's famous sheep sweater are listing it for auction this fall.
• In an email to Insider, they said they thought they'd accidentally given it away decades ago.

Princess Diana's famous sheep sweater will be auctioned this fall — but where had it been all these years since she wore it?

Sally Muir and Joanna Osborne — the garment's designers and the founders of its fashion brand Warm & Wonderful — didn't have the answers to that question until March of this year.

In fact, they thought they'd accidentally given the sweater away decades prior.

"We thought we'd probably sent it out as part of an order, as we were always having trouble fulfilling orders in those days, due to the increase in sales — due to Princess Diana," Muir and Osborne said in an email sent to Insider. "We didn't think we still had it, so we weren't looking for it."

According to a press release from Sotheby's, the original red garment was damaged by Princess Diana and sent back to Warm & Wonderful weeks after she first wore it to a polo match in 1981.

At the time, she'd requested a repair or replacement, so the two designers made her a new one, which she wore to another polo match in 1983.

The original stayed with the designers, and remained missing until a few months ago.

While searching for a sewing pattern in an attic, Muir and Osborne discovered the sweater tucked into a box with a cotton bedspread. Immediately, the two felt "surprise and disbelief."

"As it had the mend on the cuff, it was clearly that particular one," they said in their email to Insider, noting they were also "hugely relieved that it hadn't been decimated by moths."

Muir and Osborne said the sweater has had an "extraordinary" impact on their lives, but that it's time to let the original go.

"We didn't want the responsibility of having to keep it anymore, so it's time to move it on to someone who can keep it safely," they said.

The sweater auction will start on August 31 and remain open until September 14. Sotheby's estimates that it will be listed between $50,000 and $80,000.