“The Duke and Duchess of Cambridge thank you for your kind words following the death of The Duke of Edinburg,” it reads.
“Their Royal Highnesses have been incredibly moved by the many thoughtful messages they have received in recent weeks.
“They will all miss their much loved grandfather and great-grandfather, but your message has provided great comfort at this difficult time.”
Charles also sent out thank you cards in May following the passing and funeral of Prince Philip.
The Prince shared a retro black and white photo of himself as a child, approximately nine or 10-years-old with his father, Philip, boating during a visit to the Isle of Wight in 1958.
The card read: "His Royal Highness has been enormously touched by the many generous messages that have been received in recent days; they have provided great comfort at this very sad time."