Boris Johnson, who left office as British prime minister earlier this week, said the queen's death has left an “ache” in everyone’s hearts and a “deep and personal sense of loss” that is “far more intense, perhaps, than we expected.”

“This is our country’s saddest day,” he said in a statement Thursday. “In these first grim moments since the news, I know that millions and millions of people have been pausing whatever they have been doing, to think about Queen Elizabeth, about the bright and shining light that has finally gone out,” Johnson said.

“She seemed so timeless and so wonderful that I am afraid we had come to believe, like children, that she would just go on and on,” he said.

Britain's new monarch is to be officially known as King Charles III, a spokesperson has confirmed.

His official name upon the death of the queen was unknown until now and was not expected to be announced so soon.

President Joe Biden mourned the death of Queen Elizabeth II on Thursday, calling her “a stateswoman of unmatched dignity and constancy who deepened the bedrock Alliance between the United Kingdom and the United States.”

In a statement, Biden and the first lady wrote that “Her Majesty Queen Elizabeth II was more than a monarch. She defined an era.”

Taoiseach Micheál Martin, the head of the Irish government, said Queen Elizabeth II's "support for the Good Friday Agreement, reconciliation and mutual understanding was critical to advancing positive relations on these islands."

"I hope that in this time of mourning the British people and her family can draw comfort and be proud of the enormous contribution, which her majesty made in fostering and strengthening the British-Irish relationship," he said, later fondly recalling her May 2011 visit to Ireland in his remarks.

The Good Friday Agreement helped bring to a close decades of political violence in Northern Ireland, often referred to as The Troubles.

Liz Truss, who only became British prime minister Tuesday, has said the queen was "the rock on which our country was built."

In a speech outside No. 10 Downing St. shortly after the news broke, Truss said that everyone was “devastated” at the news, which was a “huge shock to the nation and the world.”

“Britain is the great country it is today because of her,” Truss added. “Her devotion to duty is an example to us all.”

"Today the Crown passes, as it has done for more than 1,000 years, to our new monarch, our new head of state, His Majesty King Charles III," she said. Britain would "offer him our loyalty and devotion, just as his mother devoted so much to us for so long," the prime minister said, ending the statement by saying, "God save the king."

Nicola Sturgeon, first minister of Scotland, where Queen Elizabeth II died this afternoon, said the monarch's death is a "profoundly sad moment" for the nation, the Commonwealth and the world.

"On behalf of the people of Scotland, I convey my deepest condolences to The King and the Royal Family," Sturgeon said in a tweet.

Queen Elizabeth II’s death comes at a precarious moment for the United Kingdom, both politically and economically. 

The queen died mere days after officially appointing the country’s latest prime minister, Liz Truss, a Conservative. The news of the queen's poor health interrupted the British Parliament as it debated the government’s plans to combat spiraling energy bills amid a growing cost-of-living crisis.

Prince William became the next in line to the British throne following Queen Elizabeth II's death Thursday. The new king is Charles, the queen's eldest son.

In a statement posted on Twitter on Thursday afternoon, Charles said the death was "a moment of the greatest sadness for me and all members of my family."

The Duke of Cambridge has not yet made a public statement on his grandmother's death. He is married to Catherine, the Duchess of Cambridge.