The handwritten notes were shared by Prince William and Kate Middleton on their official Kensington Palace Instagram account on Sunday, to mark the special day.
In the first letter, seven-year-old Prince George wrote: "Dear Granny Diana, Happy happy Mother's Day. I love you very much and think of you always, sending lots of love from George xxx."
VIDEO: The Cambridge children enjoy special outing with their parents
The little boy had also drawn Diana a beautiful picture of a sunset, trees and birds flying in the sky.
The second letter, accompanied by love heart and stickers, was written by Princess Charlotte, five. The little girl wrote: "Dear Granny Diana, I am thinking of you on Mother's Day, I love you very much. Papa is missing you. Lots of love Charlotte xxx."
The third letter featured an adorable love heart drawing from two-year-old Prince Louis, which was accompanied by his name.
The post was accompanied by the message: "This year Mother’s Day will be different once again.
The Duke and Duchess of Cambridge's children wrote letters to Princess Diana on Mother's Day
"Many of us will be apart from our loved ones, but looking forward to a time in the not too distant future when we can give our mother a hug again.
"But for those experiencing bereavement, today may be particularly challenging.
"Each year on Mother’s Day, George, Charlotte and Louis make cards remembering their Granny, Diana, for William.
"Whatever your circumstances, we are thinking of you this Mother’s Day."
Princess Diana with a young Prince William
Fans were quick to comment on the sweet gesture, with one writing: "This is a beautiful way to honour Diana," while another wrote: "What a lovely tribute, thanks for sharing these."
A third added: "What beautiful artwork! Creative and full of love, what every grandparent loves. What a special way to make grandparents part of our lives even if they aren’t with us anymore. Beautiful."
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