The Duchess met Fabian in 2011
"The Duke and Duchess are sad to hear the news and their thoughts are with Fabian's family." said a royal spokesman.
On Saturday Fabian's parents, Darrell and Lydia Bate, spoke about how their son had taken stregnth from the letter Kate had sent him and said that he will be laid to rest with it.
"Just having the memory that the future King and Queen are rooting for you is pretty awesome to have in your armoury and for keeping your child going in the dark times," Darrell told the Mirror. "I want Kate to know that it wasn't just a letter, it was a message that meant so much. It has stayed with him and it is staying with him now that he is departed.
Fabian will be laid to rest with letters Kate sent to him
"It is the only one in the world, it has that uniqueness – it is very personal and special," he added. "It was synonymous with what Fabian went through, he would not have got it otherwise. It would be amazing to think that Kate and William knew that the letter she wrote three years ago is now sitting next to his body at rest."
Shortly after her visit to the Surrey hospital in 2011 the Duchess penned a letter of admiration to Fabian saying how much she had enjoyed meeting him and that she was touched by his "strength of character" and his "positive energy".
Fabian was diagnosed with leukaemia in 2006 and underwent three years of chemotherapy and went into remission in 2009. But sadly his cancer returned in July 2011 and he had to undergo transplants with bone marrow donated by his sister Cassia.