Britain's longest-reigning monarch Queen Elizabeth II has died aged 96 after reigning for 70 years, Buckingham Palace said on Thursday.

"The Queen died peacefully at Balmoral this afternoon," Buckingham Palace said in a statement. "The King and The Queen Consort will remain at Balmoral this evening and will return to London tomorrow."

Prince Charles immediately became King upon the death of his mother, Queen Elizabeth. The statement referred to Camila as the Queen Consort.

As new King, Charles has released a statement mourning his mother's death, calls it "a moment of the greatest sadness for me and all members of my family." 

 The flag at Buckingham Palace has been lowered to half staff following the queen's death.

Buckingham Palace announced, on Thursday, that Queen Elizabeth was under medical observation in Balmoral after physicians became worried about her health.

All four of her children -- Prince Charles, Princess Anne, Prince Andrew, and Prince Edward -- as well as her grandchildren Prince William and Prince Harry, traveled to Balmoral to be with the Queen, who devoted her life to royal duties.

Queen Elizabeth was the first British monarch in history to achieve her Platinum Jubilee, and she died barely three months after the country celebrated her 70 years on the throne in June. She was also the world's longest-reigning and longest-serving ruler.