The phone call is a part of a new documentary about her life.
Prince William and Prince Harry have opened up about what they consider their biggest regret — the last phone call with their mother, Princess Diana, before her death in 1997.
Read: Prince George's Formal Wear Steals the Show as Royal Family Arrives in Poland
William was 15 years old at the time. His younger brother Harry was 12. They were on a vacation in Scotland when they received a phone call from their mother, which they cut short because they wanted to play with their cousins.
Princess Diana was in France with her boyfriend, Dodi Fayed. Both were killed in a tragic car crash in a Paris tunnel on Aug. 31, 1997.
Twenty years after her death, the moment is being remembered in a new documentary directed by Ashley Gething called Diana, Our Mother: Her Life and Legacy.
“They told us before the interviews that they had never spoken about her at length before, and would never speak about her at length again,” Gething said of the princes.
In the HBO documentary, William and Harry are sharing never-before-seen personal photographs from their family album, which include Prince Harry with Princess Diana on the royal yacht and the brothers dressed as police officers.
Elton John is also seen recalling memories with the princess in the documentary.
Read: Don't Make Me Go! Princess Charlotte Throws a Tantrum on Last Day of Royal Tour
“She had this incredible ability to make people feel at ease and make people feel that everything is going to be all right,” he says.
The documentary premieres Monday night on HBO.
Watch: See Duchess Kate Conduct Symphony During European Tour
