The death of the Queen is known in official circles in the United Kingdom as Operation London Bridge, the protocol for which has been triggered when Buckingham Palace announced the death of the 96-year-old monarch on Thursday evening.
It set in motion a series of planned events, accompanied simultaneously by Operation Spring Tide -- the codename for Queen Elizabeth II's son and heir Charles succeeding her on the throne as King Charles III.
'London Bridge is down' is reportedly the way in which the death of the Queen would have been communicated to the Prime Minister, Liz Truss, by the Queen's private secretary.
The Foreign, Commonwealth and Development Office (FCDO) Global Response Centre is in charge of sending the news to the 15 governments outside the UK where the Queen is Head of State, and to the other 38 countries of the Commonwealth.
A footman was seen walking across the forecourt of Buckingham Palace to pin the 'Official Notification' of the Queen's death to the gates, outside which there was an outpouring of emotions by crowds gathered despite the rainy weather.
The Royal Family is set to publish official plans for the funeral, which is expected 10 days after her death on Thursday.
A national mourning and minute's silence will be announced after Prime Minister Truss meets with the new King.
King Charles III will deliver a broadcast to the nation and a 'spontaneous' Service of Remembrance will be held for the prime minister and a small number of ministers at St Paul's Cathedral in London.
An Accession Council is the one that proclaims King Charles as the new sovereign, which will be read at St James's Palace and the Royal Exchange in London.
The Parliament will meet to agree on a message of condolence and for MPs to give tributes in the House of Commons. All parliamentary business will be suspended for 10 days.
This will be followed by the Prime Minister and Cabinet meeting with the new King.
As the Queen died at Balmoral Castle in Scotland, her body will be carried to London by royal train dubbed Operation Unicorn or Operation Overstudy will kick in for her body will be flown back to London.
Operation Lion refers to the Queen's coffin being transferred from Buckingham Palace to the Palace of Westminster via a ceremonial route through London. When it arrives, a memorial service is expected at Westminster Hall.
The coffin is expected to lie in state where hundreds of thousands of people are expected to converge on London to visit her and pay their respects, and condolence books will be opened online.
The State Funeral is expected at Westminster Abbey in London.
No official details have been released about what will happen over the coming days, but it is anticipated that the Queen will be given a full state funeral, as is traditional to mark the death of a monarch.
It is also expected that her body will lie in state to allow the public to pay tribute.
King Charles III will sign off the final plans in the coming days.