The Daily Telegraph reported the 26-year-old royal made the comments Thursday as he accepted his appointment as patron of the Child Bereavement Charity, a group his late mother had been associated.
"My mother Diana was present at your launch 15 years ago, and today I am incredibly proud to be able to continue her support for your fantastic charity, by becoming your royal patron," Prince William said. "Never being able to say the word 'Mummy' again in your life sounds like a small thing. However, for many, including me, it's now really just a word -- hollow and evoking only memories."
The prince's speech was delivered as the charity began its "Remember on Mother's Day" campaign, which was created to raise awareness of the problems faced by mothers who have lost children or children whose mothers have died, The Telegraph said.
"I can ... wholeheartedly relate to the Mother's Day campaign as I too have felt -- and still feel -- the emptiness on such a day as Mother's Day," the prince told the crowd.