Why the dress Princess Diana wore for her first royal engagement caused a stir

If there's one aspect of the British monarchy that hasn't changed in recent decades, it's the intense scrutiny over the wardrobe choices made by royal women.

As well as contending with the public's opinions of their clothing, royals are also faced with expectations from within 'The Firm' as to how they should dress.

Though Princess Diana is today remembered for her royal style moments, even she fell foul of the monarchy's preferences — on her very first official outing, no less.

On March 3, 1981, Diana, then 19, embarked on her first royal engagement since her engagement to Prince Charles was announced days earlier.

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Attending a musical recital at Goldsmiths' Hall in London, she wore a black taffeta ballgown by David and Elizabeth Emanuel, who also designed her wedding dress.

Speaking to biographer Andrew Morton years later, Diana recalled being "so excited" ahead of the event and pleased with her choice of outfit.

"I got this black dress from the Emanuels, and I thought it was OK because girls my age wore this dress," she is quoted saying in Diana: Her True Story - In Her Own Words.

"I hadn't appreciated that I was now seen as a royal lady, although I'd only got a ring on my finger as opposed to two rings."

Speaking on The Windsors podcast, 9Honey royal commentator Victoria Arbiter added: "She just looked young and pure and innocent, and everyone sort of just wanted to take care of her."

Diana's dress was certainly eye-catching, and adhered to the gala event's black tie dress code, but it wasn't exactly in line with the royal family's sartorial guidelines.

The royal bride-to-be had believed black a wise colour choice, telling Morton: "To me, [it] was the smartest colour you could possibly have at the age of 19."

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However, Prince Charles promptly informed her the royals had a very different view of when it was appropriate to wear the shade.

"I remember walking into my husband-to-be's study, and him saying: 'You're not going in that dress, are you?' I replied: 'Yes, I am'," she recalled to Morton.

"And he said: 'It's black! But only people in mourning wear black!' And I said: 'Yes, but I'm not part of your family yet.'"

The cut of the dress, which featured a strapless neckline, also proved troublesome for Diana.

"It was a real grown-up dress. I was quite big-chested then and they all got frightful excited," she told Morton, adding the gown had been "two sizes" too small.

Designer Elizabeth Emanuel later explained in her book A Dress for Diana that she had no idea the design would cause "such a furore".

"We hadn't considered the fact that when Diana bent over - as she would have to do when getting out of the car - she would show quite a lot of cleavage," Emanuel admitted.

"We just thought she looked fabulous."

LISTEN: 9Honey's royal podcast The Windsors delves into Princess Diana's lingering impact on the British royal family. (Post continues.)

Though it made headlines the following day for the wrong reasons, Emanuel still believes the dress heralded Diana's "astounding" transformation into a royal style icon.

"One minute she was a young nursery school teacher and the next she was a fully-fledged princess in waiting," she wrote.

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"Here was a beautiful woman in a beautiful dress escorting the heir to the throne. Prince Charles had finally met his princess."

Sartorial discomfort was only one aspect of the future Princess of Wales' woes during that "horrendous" first royal engagement.

Diana told Morton she also struggled to stay across the various elements of royal 'protocol' during the event.

"I didn't know whether to go out of the door first. I didn't know whether your handbag should be in your left or right hand," she recalled.

"I was terrified, really, at the time everything was all over the place. I remember that evening so well."

During the event, Diana encountered a fellow guest who understood the pressures of adapting to royal life: Princess Grace of Monaco.

The former Hollywood actor, then 52, took Diana aside and listened as she spoke of her struggles to fit in to her new world.

"Don't worry," Grace joked while patting Diana's cheeks, "it will only get worse."

Though her life in the royal family certainly wasn't a walk in the park, the former school teacher did quickly master the art of dressing like a princess.

Many of her outfits, including the midnight blue gown she wore while dancing with John Travolta at the White House, are still held up as examples of iconic royal style.