Princess Diana left Prince William and Prince Harry with an important warning before her death in 1997, her former private secretary Patrick Jephson exclusively tells PEOPLE.

“We all remember the days when Harry and William were joshing with one another, and it all seemed set for their relationship and the future—that Harry, as Diana always used to say, would be William’s wingman,” Diana’s biographer Andrew Morton told People. “It’s a great loss for the monarchy.”