Fittingly, the plan in place (which has been in development since the 1960s) is called Operation London Bridge. According to the plan, the Queen's private secretary, Edward Young, is the first to know of her passing from the Queen's personal physician, gastroenterologist Huw Thomas. Young then alerts the prime minister with code words, "London Bridge is down." When the Queen's father, George VI, died 70 years ago, the code was "Hyde Park Corner".
The Foreign Office's Global Response Centre then alerts nations in the Commonwealth before the media and general public are told.
Internally, the day of the Queen's death is referred to as "D-Day", and each following day leading up to the funeral is "D+1", "D+2", and so on.