Talking about how he viewed, and still views, their relationship, Mr Harrold compared William and Kate to the late Queen Elizabeth II and Prince Philip.
As reported by People, he said at an event in London on Monday: “When you saw them together and see them get on as friends, although they were in love, they are also absolutely best friends.
“They remind me of the Queen and Prince Philip. Their relationship was similar. They were best friends who worked together as a team.”
Prince William and Princess Kate were believed to have been friends before they started dating at university over 20 years ago.
Speaking in their 2010 engagement interview, William, 43, said that their relationship blossomed when they lived together at university as friends, along with two others.
The couple marked their 14th wedding anniversary earlier this year.