Pope Francis has sent a telegram to King Charles, offering his prayers for him and all those who mourn the death of Queen Elizabeth II.
The pontiff said he was deeply saddened to learn of her passing.
"I willingly join all who mourn her loss in praying for the late Queen's eternal rest, and in paying tribute to her life of unstinting service to the good of the nation and the Commonwealth, her example of devotion to duty, her steadfast witness of faith in Jesus Christ and her firm hope in his promises."
The Pope first met the Queen in 2014 as part of celebrations for the 100th anniversary of the re-establishment of diplomatic relations between the United Kingdom and the Vatican.
Francis has also met King Charles on a number of occasions, often speaking of their shared concern for the environment.
In his message to the new monarch, the pontiff finished: "I assure Your Majesty of my prayers that Almighty God will sustain you with his unfailing grace as you now take up your high responsibilities as King.
"Upon you and all who cherish the memory of your late mother, I invoke an abundance of divine blessings as a pledge of comfort and strength in the Lord."