The dress that caused a scandal months before Princess Diana's wedding: 'I don't think she realised'

A dress worn by Diana in her first outing as a royal fiancée propelled her into the spotlight for all the wrong reasons.

It also saw the beginning of Diana as a global fashion icon, willing to break the rules and ignore royal protocol.

But Prince Charles took issue with the gown because of its colour, not how much it revealed of his future bride.

It was March 9, 1981, just months before the royal wedding.

Prince Charles had announced his engagement to Lady Diana Spencer in February and the couple was making their public debut.

It was Diana's first official outing with the Prince of Wales after their intention to marry was made public by Buckingham Palace.

The couple were the guests of honour at Goldsmiths Hall, in aid of The Royal Opera House.

Monaco's Princess Grace was also present at the ball.

Diana wore a black strapless taffeta gown, which proved to be quite low-cut. It was designed by David and Elizabeth Emanuel, who went on to design Diana's wedding dress.

In a newsreel from the next day, a reporter says of the future Princess of Wales: "The palace did not know in advance of the dress she chose to wear…by normal royal standards, it was fairly revealing".

READ MORE: Inside the fairy tale engagement and wedding of Princess Diana, 40 years on

One of the photographers covering the event was Ken Gavin, former chief photographer for The Mirror.

He was stationed at the top of the stairs, ready for Prince Charles and Diana to walk up towards the press pack.

"When she came out in that black dress, cleavage and everything [nearly] falling out everywhere, she was, she looked sensational," Gavin told the new three-part documentary Diana's Decades, which aired overnight.

"She came up the stairs and we were waiting for her. I knew that I had a page-one picture, there was no question about that.

"I think she would have known she was going to get a lot of attention [but] I don't think she realised how low that dress was until she saw the stills the next day.

"Wow. The royal family – haven't seen them like that very often."

The resulting footage captured by the cameras that night were predominantly taken from above, meaning the press got to see more of Diana than anyone had ever thought.

But Prince Charles didn't seem fussed by the dress's revealing nature. He was concerned about its colour.

Princess Diana later told her biographer Andrew Morton how Charles reacted when he first saw the gown, moments before they were due to leave for the ball.

READ MORE: How Princess Diana used the media to her advantage: 'That was her power'

"I remember my first engagement so well," Diana told Morton. "So excited. I got this black dress from the Emanuels and I thought it was OK because girls my age wore this dress.

"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. 

"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.' 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.'

"Black, to me, was the smartest colour you could possibly have at the age of 19. It was a real grown-up dress. I was quite big-chested then, and they (the Press) all got frightfully excited."

Diana told Morton how it was Princess Grace who helped her cope with the newfound attention.

"I learned a lesson that night. I remember meeting Princess Grace and how wonderful and serene she was," Diana said.

"'Don't worry,' Princess Grace joked. 'It will only get worse.'

"It was a horrendous occasion. I didn't know whether to go out of the door first. I didn't know whether your handbag should be in your left hand or your right hand.

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