Speaking about the emotional trip to the grave, the Daily Mirror quoted a source as saying, "It was very important for William to take Kate to visit his mum just before their wedding day."
The source added, "Diana is still a huge part of her boys' everyday life and always will be. It is tragic that she won't be there to see the wedding and that she never got to meet his bride...But even though Kate never met Diana she knows what an incredible woman she was and it is very important to her she can share and understand William's love and grief for his mum."
Highlighting the importance of his late mother in his life, Prince William had presented Kate Middleton with Diana's diamond and sapphire engagement ring when he proposed her for marriage.
"It was my way of making sure my mother didn't miss out on the excitement, and the fact that we're going to spend the rest of our lives together" the Prince said.
Princess Diana, still a hugely popular figure died in 1997 at the age of 36 in a Paris car crash.