The Queen, 94, has been on the throne for the past 68 years – surpassing the reign of 63 years, seven months and two days set by Queen Victoria. Princess Elizabeth became Queen on February 6, 1952, with the coronation taking place on June 2, 1953.
The Queen ascended to the throne following the passing of her father King George VI and contingency plans for her own death have been in place ever since the 1960s.
An operation known as “London Bridge is down” will be activated around the world when the sad day arrives.
Messages will immediately be sent out to the Prime Minister as well as the Commonwealth nations where the Queen is head of state.
Buckingham Palace does not comment on arrangements for the funerals of members of the Royal Family, but over the years more and more details of plans have emerged.
The Government, civil servants and royal aides meet around three times per year to update plans.
The death of a head of state is set to lead to a 12-day period of national mourning across the UK.
Every eventuality, including what to do in any possible location where the Queen passes away, has been meticulously thought out.
However, in every scenario it is understood the Queen will be brought back to the throne room in Buckingham Palace.
But, on her 21st birthday in 1947, Princess Elizabeth vowed to dedicate her “whole life” to serving her country.
Meanwhile, earlier this week ministers announced plans to mark the Queen’s 70th year as head of state in 2022.
The Platinum Jubilee celebrations will see Britons granted an extra bank holiday as part of a four-day weekend from June 2.
Culture Secretary Oliver Dowden said it will be “a truly historic moment, and one that deserves a celebration to remember”.