London based journalist Jesse Peach explains what was the reaction on London's street over the Royal announcement.
The world went into a frenzy yesterday when royal staff from across the UK were summoned to London for a "highly unusual" meeting.
The news sparked another bout of feverish speculation over whether the 91-year-old Queen or her husband Prince Philip, 95, had died.
Buckingham Palace had to reassure the public that both were alive and well, and a few hours later, the less dramatic news emerged that the Duke of Edinburgh will be taking a step back from public duties.
A recent Guardian feature revealed the secret, intricately detailed plan in place for when the Queen dies, known as "London Bridge is Down", but the protocol for the older Prince Philip is less well known - and very different.
The news will be confirmed first by the BBC, and if he dies at night, the announcement is likely to come at 8am local time.
While the Queen's consort would be entitled to a full state funeral, he has long insisted he does not want the "fuss".
His body is expected to first lie in St James's Palace, where Princess Diana was laid, instead of Westminster Hall at the Houses of Parliament. The public will not be allowed to view the body.
The funeral will be low key. Prince Philip's family, friends and some Commonwealth countries' heads of state will attend a service at St George's Chapel in Windsor Castle. He will then be buried in Frogmore Gardens in the castle grounds.
The Queen and staff at the Palace will enter a period of mourning lasting approximately eight days, the Daily Beast reports, during which the monarch - assuming she outlives her husband - will stop work. She will not be available to give the Royal Assent to new laws, and affairs of state will be put on hold.
Flags at major institutions and military establishments will be flown at half-mast, but the Royal Standard flag flown at the Palace when the Queen is in residence will not be lowered to half-mast. Despite a rumour circulating yesterday, the Royal Standard is never flown at half-mast, even after the demise of the Crown, as there is always a sovereign on the throne.
The official period of Royal Mourning will end 30 days later, and it is expected that the Queen will resume her duties, unlike Queen Victoria, who locked herself in Balmoral after Albert's death and was rarely seen again.
A 1945 file photo of Prince Philip in Melbourne, Australia. Photo / AP
'LONDON BRIDGE IS DOWN'
In the event of Queen Elizabeth's death, Sir Christopher Geidt, the Queen's private secretary, will be the first official to convey the news. Geidt will call the prime minister and use the secret code "London Bridge is down" to inform of her passing.
It's a tradition that comes from the fact previous palace officials used a code to make sure switchboard operators weren't the first to find out. When King George VI (the last British monarch to pass) died in 1952, the code that palace workers used was "Hyde Park Corner."
Queen Elizabeth's secret code is "London Bridge is down." This statement will kick off Operation London Bridge, the plan to alert the world that Queen Elizabeth II has died.
The statement will kick a complex and sombre operation into gear. After the prime minister is alerted, the Foreign Office's Global Response Center will inform the 15 governments where the Queen is still head of state, including Australia, Canada, the Bahamas and Belize. Then the 36 nations where she still serves as a figurehead will be informed.
Next, the Press Association will send out a news alert to simultaneously notify the world's media - a break from the past, when the BBC was always the first to know about royal deaths. Social media and modern technology have negated that system.
Radio stations in the UK have blue "obit lights" that go off in such events. DJs have sombre playlists cued up for these occasions and will know to play inoffensive music before switching to news.
A footman in mourning clothes will pin a notice to the Buckingham Palace gates, as the palace's website is updated with the statement on a dark background.
Prince Charles becomes King Charles, making Prince William the Prince of Wales and Catherine, Duchess of Cambridge, the Princess of Wales - Princess Diana's former title.
Great Britain will enter an official 12-day mourning period, with the Queen's funeral taking place at the end of the two weeks.
