Queen Elizabeth death LIVE Updates: King Charles III pledges to uphold constitutional principles in his first speech

British Prime Minister Liz Truss paid tribute to the late Queen Elizabeth on Thursday, while offering support and loyalty to the nation's new monarch Charles. "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," she said outside her Downing Street office.

"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," Charles said.

It is no secret that the UK's plan for the eventual death of Queen Elizabeth II was codenamed London Bridge. But there were special provisions if the monarch died when she was in Scotland, called Operation Unicorn. The unicorn is the national animal of Scotland and forms part of the royal coat of arms, along with the lion of England. The 96-year-old monarch passed away at her remote Highlands residence, Balmoral, with all her immediate family flying in to be at her side.

The British monarchy’s rules state that “a new sovereign succeeds to the throne as soon as his or her predecessor dies.” That means Queen Elizabeth II’s eldest son, Prince Charles, became king immediately upon her death. However, it may be months or even longer before Charles’ formal coronation. In Elizabeth’s case, her coronation came on June 2, 1953 -- 16 months after her accession on Feb. 6, 1952, when her father, King George VI, died.