Princess Diana upset the Queen by wearing this heirloom necklace as a headband

The Princess always pushed the envelope when it came to style.

It's long been known that Princess Diana was a rule breaker when it came to her sartorial choices. From the 'revenge dress' to opting not to wear gloves on official visits, Diana did it her way when it came to royal dressing.

Unfortunately, not all were in agreement with some of her more creative style choices – with one of Diana's most iconic looks reported to have rubbed the Queen the wrong way.

The look in question? Diana's innovative use of a royal family heirloom necklace as a headband when she attended a gala dinner in Melbourne, Australia, with then-husband Prince Charles.

Diana commemorated the 1985 visit by pairing a teal satin evening gown made by her wedding dress designers, David and Elizabeth Emanuel, with the heirloom piece of jewellery: an art deco, emerald choker once belonging to Queen Mary, that the Queen had passed to the princess upon her joining the royal family.

The decision to wear the choker as a headpiece, however, didn't go down too well with the monarch.

'What Diana was wearing while dancing around in her husband's arms was actually a necklace that was part of the Crown Jewels that she had been given by Queen Elizabeth herself,' royal commentator Debora Attanasio told Marie Claire Italia (as reported by Express).

'It was a piece that the Queen was very fond of, but which she, in turn, decided to pass on to Diana as a wedding gift,' Attansio said.

'According to sources close to the Queen, having seen what Diana had done with it, Her Majesty became very irritated.'

But the unusual placement of the choker might not have been Diana's idea – with the historic look reportedly coming about by happy accident.

Royal biographer Kitty Kelley claimed in her book The Royals that in a rush, Diana tried to put the choker over her head, rather than wait for someone to help her clasp it. When it got stuck and wouldn't go over her nose, Canadian diplomat Victor Chapman convinced her to wear it as a headpiece instead.

'Diana took the necklace and put it over her head rather than wait to have it clasped around her neck. She couldn’t get it over the bridge of her nose,' Kelley wrote. 'Vic roared. "Leave it there," he said, "It’s young and fun, like you".'

And so there you have it: one of Princess Diana's most memorable looks. And we have to say, we're firm fans of the jewellery trick – even if the Queen was decidedly less so...