Watch: Queen dies aged 96: Flowers laid outside Windsor Castle
Preparations known as "Operation London Bridge" have been updated for many years, but because she died in Scotland a second plan called "Operation Unicorn" has also been activated.
The code phrase, “London Bridge is down”, will have been used to communicate to prime minister Liz Truss, as well as key officials, that Her Majesty has passed away.
This phrase has set into motion the plan of action around her death, including a period of official mourning and the details of her state funeral.
While the full details have yet to be officially confirmed by Buckingham Palace, here's what is known about how the coming days could unfold.
'London Bridge' and 'Unicorn'
The plans were initially created in the 1960s and involved a number of government departments, as well as the Church of England, the Metropolitan Police, the British Armed Forces, the media, the Royal Parks and Transport for London.
Royal funerals are organised by the Earl Marshall and officers in the College of Heralds. Codenames for the death of a royal are nothing new.
The death of the Queen’s father, King George VI, in 1952, operated under the code phrase, “Hyde Park Corner”. Plans for the Queen Mother’s death in 2002 were called “Operation Tay Bridge”, while “Operation Forth Bridge” referred to the death of the Queen’s husband, Prince Philip, the Duke of Edinburgh, who died in 2021. “Operation Menai Bridge” refers to the funeral plan for her son, Charles.
The moments after the Queen’s death
Under Operation London Bridge, the first official – apart from the Queen’s family and medical team – to convey the news of her death would have been her private secretary.
Their first act was to tell the prime minister, who was given the news through a secure telephone line by civil servants using the code phrase, “London Bridge is down”.
The Queen’s private secretary was also responsible for informing the cabinet secretary and the Privy Council Office.
Government ministers and senior civil servants were informed by the cabinet secretary, and the Foreign Office told the 14 Commonwealth countries where the Queen was head of state.
Today is referred to internally by those organising it as “D-Day”.
The media reaction
The government’s websites and social media accounts have already turned black to mark the Queen’s passing, and no content that is non-urgent will be published there.
The BBC and the PA Media news agency learned of the Queen’s death through the Radio Alert Transmission System (RATS).
Commercial radio were notified through the Independent Radio News via a network of blue “obit lights”, which let DJs know to prepare soothing and inoffensive music while they waited for a news flash.
BBC Two suspended its scheduled programming and switched to BBC One’s broadcast of the announcement of the Queen’s death.
A pre-recorded sequence of portraits of the Queen broadcast on BBC News allowed presenters to change into black clothing to be ready for the formal announcement.
The political reaction
A royal footman has pinned a dark-edged notice to the gates of Buckingham Palace and that notice is also displayed on the palace website.
Prime Minister Liz Truss addressed the nation from outside Downing Street just under an hour after the Queen's death was announced.
She said: "It’s an extraordinary achievement to have presided with such dignity and grace for 70 years. Her life of service stretched beyond most of our living memories.
"In return, she was loved and admired by the people in the United Kingdom and all around the world."
She added: "Today the crown passes, as it has done for more than 1,000 years, to our new monarch, to our new head of state, His Majesty King Charles III."
The king will host a meeting with the prime minister and then deliver a speech as soon as possible.
Flags are being flown at half-mast on Whitehall and government buildings. A service of remembrance, attended by the prime minister and senior government ministers, will take place at St Paul’s Cathedral in London as soon as possible.
The new monarch
Tomorrow, under Operation London Bridge, the Accession Council will meet at St James’s Palace to proclaim the new monarch, King Charles.
The plan for Charles’s accession has its own code name, “Operation Spring Tide”.
The Privy Council must meet within 24 hours of the Queen’s passing, and will proclaim Charles as king in the first part of the meeting. He may or may not be there for this part, but his presence is required for the second section, in which the new monarch reads a declaration and take an oath, assuming the duties and responsibilities of the sovereign.
Before her death, the Queen said she wanted Camilla, Duchess of Cornwall, to be known as queen consort when Charles became king.
A special session of Parliament will be held as soon as possible where MPs will swear allegiance to the King, after which most parliamentary activities will be suspended for a period of time.
The King will host the prime minister and the cabinet for an audience as soon as possible.
Once this is done new monarch will be made by the Northern Irish, Scottish and Welsh administrations.
Charles is expected to make a tour of the UK in the coming days visiting Scotland, Wales and Northern Ireland.
The Queen’s final journey
Because the Queen died at Balmoral Castle in Scotland, the plan under Operation Unicorn has been activated.
This means her coffin is expected to lie in repose at Holyrood Palace followed by a service of reception which will likely be at St Giles’ Cathedral in Edinburgh.
The Queen’s coffin will then be taken by plane to the Throne Room at Buckingham Palace and finally, it will be taken to Westminster Hall and lie in state.
At Westminster Hall, the Queen’s coffin will be placed on a raised platform, called a catafalque, and members of the public will file past to pay their respects.
It's expected the hall will be open for 23 hours a day to cope with the numbers, with a brief closure for cleaning each day.
The Vigil of the Princes, in which the Queen’s children and grandchildren arrive unannounced to stand watch over the coffin, is also expected to take place.
The Queen’s state funeral will be held at Westminster Abbey, under Operation London Bridge.
The service will be carried out by the Archbishop of Canterbury.
After the funeral, the Queen's body will be buried in a prepared tomb at King George VI Memorial Chapel at St George’s Chapel at Windsor Castle, alongside her husband, Prince Philip, whose coffin will be moved from the Royal Vault.
The day of the Queen’s funeral will be a day of national mourning, but a bank holiday will not be granted.
There will be a two minutes’ silence across the UK at midday and there will be processions in London and Windsor.