After speculation that Queen Elizabeth II's heir might have taken a different regnal name, Britain's new king will be formally known as Charles III, his Clarence House residence confirmed Thursday.

William, Charles' eldest son, inherits the duchy of Cornwall in addition to his current title of Duke of Cambridge.

Britain's new King Charles

Britain's new king mourned his mother Queen Elizabeth II and said on Thursday she was a "cherished sovereign and a much-loved mother" who would be missed around the world.

Buckingham Palace released an official statement after Charles, 73, ascended the throne following his mother's 70-year reign:

"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."

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"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."

"During this period of mourning and change, my family and I will be comforted and sustained by our knowledge of the respect and deep affection in which the queen was so widely held."

"God Save the Queen"

As news of Elizabeth II's death broke on Thursday, tearful crowds outside Buckingham Palace sang a forlorn "God Save the Queen."

Hundreds braved torrential rain to gather outside the palace gates after doctors announced that the monarch had been placed under medical supervision at Balmoral, her Scottish estate.

A bright rainbow temporarily lifted spirits before the seismic news was announced at 6:30 p.m. (1730 GMT), prompting widespread cries of "oh no", according to AFP.

The Union Jack flag on the queen's London residence was lowered before a silent mourning crowd. 

"She's been the queen for as long as I've been alive, she's been the queen for as long as my parents have been alive," currency broker Charlie Wolstenholme told AFP.

"So she's really a very, very important part of the fabric. You know, it's going to be terrible."

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Since last October, Britain's longest-serving monarch had been plagued by health issues that made it difficult for her to walk and stand. Many of the well-wishers, some carrying flowers, came from outside the UK.

"As a French person, even I am touched by this," said student Chloe Papeil. "She is a part of English culture, but also global culture."

The Queen, who is instantly recognizable to billions of people around the world, was celebrating her Platinum Jubilee, which marked 70 years since she succeeded her father, King George VI, in 1952.

The news spread quickly across the country, with announcements in public places, including a train from London to Edinburgh.