When Prince Philip first met Diana, who was 19-years-old at the time, the impact of the meeting saw him write a letter to Prince Charles.
According to author Gyles Brandreth, her shyness and youth led the Duke of Edinburgh to give "an ultimatum" to his son Prince Charles regarding marriage.
Gyles, while writing for Mail Online, revealed Prince Philip’s involvement in the couple’s relationship in its early stages.
He wrote: “Diana, of course, had proved to be a master at playing to the gallery.
“Yet when Philip first met her as a shy 19-year-old, she’d brought out his protective instincts.
“With Press speculation at fever pitch in early 1981 about a possible wedding, the Duke decided it was time to interfere.”
Prince Philip wrote a letter to Prince Charles speaking about how he should proceed to marrying Diana.
Brandreth added: "In a letter to his 32-year-old son, he pointed out that the girl was young and vulnerable, and it was time for Charles to put up or shut up.
"He should either propose to Diana, counselled his father, so 'pleasing his family and the country', or release her.
"But he really shouldn’t let her go on dangling in the wind."
The letter intended to make Charles feel"almost compelled to propose".
"It wasn’t a bullying letter at all. It was very reasonable. It was fair. It was sensible," Gyles said.
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