Princess Diana, Prince Charles' first impression of each other revealed

Prince Charles and Princess Diana on this day 40 years ago had made the world know about their engagement.

It is said that they met just 13 times,  within six months of their courtship, but what the public was not aware about was that Charles and Diana were already familiar with each other by the time they began dating.

Their first interaction happened in 1977 when he was going out with her older sister while Diana was just 16.

Charles recalled that she "very jolly and amusing", while Diana found him "pretty amazing".

During their official engagement announcement, he told reporters that he had asked Diana to marry him three weeks earlier.

This was prior to Diana's trip to Australia and the Prince of Wales deliberately chose that moment so she could think it over.

"I thought it would be a good idea, that ... she could then think about it, and if she didn't like the idea, she could say she didn't," he said.

However, Diana made up her mind and accepted the proposal on the spot without hesitation.

"It wasn't a difficult decision in the end," she said.