The couple has recently paid an emotional visit to the late Princess of Wales' final resting place on an island at her family's Althorp estate in Northamptonshire, and laid flowers at her grave, the Daily Express reported.
St James's Palace, however, refused to discuss the visit, saying it was a private matter.
It is understood that during their trip to the Spencer family's 14,000-acre estate, William, 28, and his 29-year-old fiancee took a boat to the island burial spot called The Oval. They also visited the memorial to Diana and the arboretum where William and his younger brother Prince Harry planted trees when they were children, sources were quoted by the British newspaper as saying.
One source said, "It was very important for William to take Kate to visit his mum just before their wedding day. Diana is still a huge part of her boy's everyday life and always will be. Even though Kate never met Diana, she knows what an incredible woman she was and it's important to her she can share and understand William's love and grief for his mum."
William and Kate have made it clear that they intend to honour Diana's memory at their wedding on April 29. He also gave Kate his mother's blue sapphire engagement ring.
"It's my mother's engagement ring so I thought it was quite nice because obviously she's not going to be around to share any of the fun and excitement of it all. It is my way of making sure my mother didn't miss out on the fact we are going to spend the rest of our lives together," William said on their engagement day announcement in November.