Prince William has paid tribute to the Queen, saying the world has "lost an extraordinary leader" while he lost his "Grannie".
The new Prince of Wales said "she was by my side at my happiest moments. And she was by my side during the saddest days of my life".
He said he would "honour" his grandmother's memory "by supporting my father, the king, in every way I can".
The prince is now first in line to the throne after his father became king.
The Queen, the longest reigning monarch in British history, died peacefully aged 96 on Thursday at her Scottish estate in Balmoral.
In a statement, Prince William said: "On Thursday, the world lost an extraordinary leader, whose commitment to the country, the realms and the Commonwealth was absolute. So much will be said in the days ahead about the meaning of her historic reign. I, however, have lost a grandmother."
He continued: "She was by my side at my happiest moments. And she was by my side during the saddest days of my life. I knew this day would come, but it will be some time before the reality of life without Grannie will truly feel real."
The 40-year-old said that while he would "grieve" the loss of the Queen he also felt "incredibly grateful" to "have had the benefit of The Queen's wisdom and reassurance" his whole life.
"My three children have got to spend holidays with her and create memories that will last their whole lives," he added.
He added: "My grandmother famously said that grief was the price we pay for love. All of the sadness we will feel in the coming weeks will be testament to the love we felt for our extraordinary Queen."