King Charles and his first wife Princess Diana announced their engagement in February 1981, but they had been keeping their news secret for weeks.
LOOK: Inside King Charles and Princess Diana's private second honeymoon – all the photos
The monarch – who is now married to Queen Consort Camilla – made the confession in their engagement interview, adding that he had planned to give Diana plenty of time to consider his proposal and the implications of joining the royal family.
WATCH: Most show-stopping royal engagement rings that will go down in history
"I asked Diana before she went to Australia – two or three days before. I thought it would be a good idea that, apart from anything else, if she went to Australia she could then think about everything," 32-year-old Charles began, before turning to 19-year-old Diana and saying: "But, in fact, you said…" Diana finished: "Yes, very promptly."
Charles continued: "Yes, then we sat on it for three weeks." The late Queen Elizabeth's eldest son proposed during a private dinner at Buckingham Palace before the future Princess picked out her beautiful diamond and sapphire engagement ring from Garrard.
DISCOVER: 11 royal couples who have divorced
READ: Prince William discusses fears about Princess Kate's family's reaction amid royal proposal
Newly-engaged Charles and his first wife Diana were pictured at Buckingham Palace in 1981
When asked when Charles spoke to his mother about his proposal plans, he replied: "Well, about three weeks ago. So, as we say, we've had to sit on it and hide it for three weeks, which hasn't been all that easy. But I was absolutely determined that it was going to be as near a secret as possible until the actual day, that was my ambition."
The then-Prince explained that they were "delighted and happy" to be engaged, joking: "I'm amazed that she's been brave enough to take me on."
The royals got married at St Paul's Cathedral in July 1981
However, a body language expert revealed Diana appeared nervous and uncomfortable in their engagement photos, which he said was an early indication that she was finding the press interest in their relationship "very daunting".
Darren Stanton, speaking on behalf of Slingo, explained: "Charles, however, was more used to being in the public eye and the expressions he is displaying in the photos don’t seem to be consistent with the emotions he is trying to convey. Instead, we can assume that Charles is just going through the motions because, unlike Diana, he was so used to having his photograph taken.
"Diana appears nervous and in some photos is seen with her arm positioned across her chest in what is known as a self-reassurance hug. This posture tends to be adopted by those that are feeling uncomfortable in their environment, suggesting Diana was nervous and quite intimidated by the level of attention she was receiving," he added.
They got married on 29 July 1981 at St Paul's Cathedral, but they finalised their divorce in 1996 after welcoming sons Prince William and Prince Harry.
Planning your wedding? Sign up to HELLO!’s Bride Guide newsletter for expert advice, budget help, wedding inspiration and real life stories delivered straight to your inbox.