Queen Elizabeth II, who ruled the UK for 70 years, died at Balmoral at the age of 96. She was the monarch who ruled the country the longest.
Concerns about her health led her family to gather at her Scottish estate on Thursday, according to BBC.
After her passing, her eldest son, Charles, the former Prince of Wales, would take the throne as the new King and ruler of 14 Commonwealth states, leading the nation in grief.
In a statement, His Majesty the King expressed: "The death of my beloved mother, Her Majesty The Queen, is a moment of the greatest sadness for me and all members of my family.
"We mourn profoundly the passing of a cherished sovereign and a much-loved mother." I know her loss will be deeply felt throughout the country, the Realms, and the Commonwealth, and by countless people around the world."
The King said that during this time of grief and change, he and the Royal Family would be "comforted and sustained" by the fact that the Queen was held in such high regard and "deep affection" by so many.
In a statement, Buckingham Palace said: "The Queen died peacefully at Balmoral this afternoon."
The King and the Queen Consort "will remain at Balmoral this evening and will return to London tomorrow," the palace statement added.
After doctors put the Queen under medical care, all her children went to Balmoral, near Aberdeen. Prince William and Prince Harry were also there.
A Committed Queen
At the age of 25, she ascended the throne and successfully led the monarchy through many storms and decades of change. During the monarchy's tougher times, her popularity helped to keep it stable, according to BBC.
Fifteen prime ministers worked for her, showing her superb knowledge of world affairs and how well she could stay politically neutral.
There were dark periods, but the passion shown on her silver, golden, and diamond jubilees proved her particular position in the nation's heart, The Guardian reported.
And she saw her job as something she had to do her whole life.
In her silver jubilee message in 1977, the Queen expressed: "When I was 21, I pledged my life to the service of our people, and asked for God's help to make that vow."
"Although that vow was made in my salad days, when I was green in judgment, I do not regret nor retract one word of it."
US President Joe Biden Extends Condolences To UK
The White House reported that earlier on Thursday, Biden talked with British Prime Minister Liz Truss about Queen Elizabeth II and expressed to her his deepest thoughts on the sick monarch.
White House spokesman John Kirby told reporters early Thursday afternoon that the president and first lady Jill Biden were "thinking" of the British royal family during a planned call with European leaders about Ukraine.
Kirby noted that Pres. Biden "did convey to Prime Minister Truss, who is on the video teleconference, that he and Dr. Biden are thinking very much of the queen and the family and the people of the United Kingdom."
The White House official added that Biden would be updated on UK developments throughout the day.
Buckingham Palace said earlier Thursday that The Queen's doctors were "worried" about her health and had "recommended" that she stay under medical supervision, as per a report from ABC News.
Queen Elizabeth has met 13 U.S. presidents in her life. Since World War II, she has met every U.S. president except Lyndon B. Johnson.
Last year, Biden went to the UK for the first time as president. While there, he met The Queen.
In 1982, while serving as a senator for the United States, he had already seen Queen Elizabeth II.
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