Her husband Prince Phillip died in April 2021 and her health noticeably shifted after the end of their 74 years of marriage.
She was hospitalised in October that year, after which she appeared for the first time in 20 years with a walking stick.
She was pictured using a walking aid at a service marking the centenary of the Royal British Legion.
In November that year, she missed the Remembrance Sunday commemorations due to a sprained back.
The following June, she did not attend a thanksgiving service at St Paul’s Cathedral on the Friday of her Platinum Jubilee.
In a statement, a palace spokesperson said: “The Queen greatly enjoyed today’s Birthday Parade and Flypast but did experience some discomfort.
“Taking into account the journey and activity required to participate in tomorrow’s National Service of Thanksgiving at St Paul’s Cathedral, Her Majesty with great reluctance has concluded that she will not attend.”
That marked what has retrospectively been understood as the beginning of a period of decline for the queen until her death in September.
Dr Douglas Glass, her official apothecary in Scotland, who holds GP clinics for Balmoral staff, said after her death that there had been concerns for the queen’s health for several months in the run-up to September.
“It was expected and we were quite aware of what was going to happen,” he is quoted as saying in Gyles Brandreth’s new biography, Elizabeth: An Intimate Portrait.
The book, which is serialised by the Daily Mail, also claimed the late monarch suffered periods of low energy while also insisting to aides she was determined to stay busy after her husband’s death in April last year.
Her biographer also writes that, when the Queen felt weaker, she would fill her days by watching BBC drama Line of Duty.
Just two days before her death, she was pictured appointing her 15th and final British prime minister, Liz Truss.
Though she had to cancel events in the final year of her life, it was reported that she routinely met with the prime minister of the time.
Her death certificate, written by Dr Glass, said the then-monarch was pronounced dead at 3.10pm on 8 September. The document was signed by the Queen’s daughter, Princess Anne.