King Charles III is expected to make an address tomorrow, a royal spokesperson told CNN.
US President Joe Biden and first lady Jill Biden released a statement on the death of Queen Elizabeth II, calling her a "steadying presence and a source of comfort and pride for generations."
"Her Majesty Queen Elizabeth II was more than a monarch. She defined an era," President Biden says in a statement.
"Today, the thoughts and prayers of people all across the United States are with the people of the United Kingdom and the Commonwealth in their grief. We send our deepest condolences to the Royal Family, who are not only mourning their Queen, but their dear mother, grandmother, and great-grandmother. Her legacy will loom large in the pages of British history, and in the story of our world," the Bidens wrote in the statement.
"Queen Elizabeth II was a stateswoman of unmatched dignity and constancy who deepened the bedrock Alliance between the United Kingdom and the United States. She helped make our relationship special," they said.
"We were honored that she extended her hospitality to us in June 2021 during our first overseas trip as President and First Lady, where she charmed us with her wit, moved us with her kindness, and generously shared with us her wisdom. ... She helped Americans commemorate both the anniversary of the founding of Jamestown and the bicentennial of our independence. And she stood in solidarity with the United States during our darkest days after 9/11, when she poignantly reminded us that 'Grief is the price we pay for love,'" the statement continued.
The Bidens said they first met the Queen in 1982 while the President traveled to the UK as part of a Senate delegation.
They said they "look forward to continuing a close friendship with The King and The Queen Consort."
US Vice President Kamala Harris and second gentleman Douglas Emhoff also offered their condolences on the Queen's passing.
"Queen Elizabeth II lived an incredible life of service and had a profound impact on countless lives and historic events. We send our condolences to the Royal Family and join the people of the United Kingdom and all those around the world who mourn her loss," the vice president's and second gentleman's Twitter accounts tweeted.
Prince William and Catherine have assumed the titles of Duke and Duchess of Cornwall, according to their official Twitter account. Prince Charles and Camilla were previously known by those titles.
Following the announcement of Queen Elizabeth II's death on Thursday, the bio for Kate and William's official Twitter account changed to: "The official account of The Duke and Duchess of Cornwall and Cambridge and the Royal Foundation, based at Kensington Palace."
Prince William and Kate previously held the title of the Duke and Duchess of Cambridge.
Queen Elizabeth's son Charles will rule under the name King Charles III, it has now been confirmed.
Tony Blair, who served as Britain's prime minister for one decade of the Queen's 70-year reign, has said "it was our great good fortune to have her reign over us."
"We have lost not just our monarch but the matriarch of our nation, the figure who more than any other brought our country together, kept us in touch with our better nature, personified everything which makes us proud to be British," Blair wrote.
Liz Truss concluded her speech outside Downing Street with a call to unite behind Charles, Britain's new King.
"We offer him our loyalty and devotion, just as his mother devoted so much to so many for so long," the UK's new Prime Minister said.
"With the passing of the second Elizabethan age, we usher in a new era in the magnificent history of our great country. Exactly as her Majesty would have wished: by saying the words, 'God save the King.'"
Watch whole speech:
Former British Prime Minister Boris Johnson tweeted a statement following the announcement that Queen Elizabeth II had died, saying it's the "country's saddest day."
"In the hearts of every one of us there is an ache at the passing of our Queen, a deep and personal sense of loss — far more intense, perhaps, than we expected," he wrote.
Prime Minister Liz Truss called on Britons to "come together" to support Charles, the United Kingdom's new King.
"In the difficult days ahead, we will come together with our friends across the United Kingdom, the Commonwealth and the world, to celebrate her extraordinary lifetime of service," Truss said.
"It is a day of great loss, but Queen Elizabeth II leaves a great legacy."
"Today the Crown passes, as it has done for more than a thousand years, to our new monarch, our new head of state: His Majesty, King Charles III," Truss said.
"With the King’s family, we mourn the loss of his mother. And as we mourn, we must come together as a people to support him," she added. "To help him bear the awesome responsibility that he now carries for us all."
“Britain is the great country it is today because of her,” UK Prime Minister Liz Truss said outside Downing Street following the death of Queen Elizabeth II.
“We are now a modern, thriving, dynamic nation. Through thick and thin, Queen Elizabeth II provided us with he stability and the strength that we needed.”
“It’s an extraordinary achievement to have presided with such dignity and grace for 70 years.”
“She was loved and admired by the people in the United Kingdom, and all round the world.
She has been a personal inspiration to me, and to many Britons, the new PM said.