Queen Elizabeth II died around 3pm on 8 September, three and a half hours before the official announcement, a British magazine has claimed.
Private Eye also claimed the monarch had been quite ill for some time and only managed to do the state appearances she did in the final months of her reign with the help of medication, despite Buckingham Palace blaming "ongoing mobility issues" for her absence.
The magazine said the Queen was "physically weak, although mentally alert".
Private Eye said the Queen had hoped to receive Boris Johnson and new PM Liz Truss for the official handover of power in London, but her doctors told her she was not fit enough to leave Balmoral.
Despite her limited mobility, she was still able to carry on her normal tasks, including reportedly managing to call her horse racing manager to check on how one of her horses was doing in the days before her death.
But she became seriously ill on 8 September, catching many of those close to her and the wider Royal Family by surprise.
Although most of her family rushed to see her as soon as it became aware she was gravely ill, in the end only Princess Anne and the former Prince of Wales were there at her bedside when she died.
Private Eye said the Queen died around 3pm, but the required phone calls to inform the wider political and media establishment were delayed until all of the Royal Family could be informed.
The magazine claims the way the Royal Family's press team handled the death of the Queen was informed by the death of Prince Philip.
They said Buckingham Palace was still claiming Philip was in "good spirits" when he was "being revived by crash teams" around the time of his death.
The Palace also struggled to keep a lid on the news of the Queen's health as members of the royal staff began briefing trusted journalists about the seriousness of her situation.
Coincidentally, BBC journalist Yalda Hakim tweeted the Queen had died around the actual time of the Queen's death, despite not knowing this and later retracting the tweet saying she had been mistaken.
On the day of the Queen's death, the nation came to a standstill almost as soon as Buckingham Palace said they were concerned about the monarch's health.
The BBC suspended normal programming from 12.30pm, with an episode of Bargain Hunt cut short.
Politicians were stunned into silence in the middle of a debate on the government's answer to the energy bill crisis.
Royals rushed to Balmoral from across the country with Prince William, Andrew and Edward arriving by car around 5pm.
Initial reports suggested the group arrived before the Queen's death, with only Prince Harry missing out on seeing her one last time, but this now appears to not be the case.
By 2pm, BBC journalists had all switched into black dress and tie.
The Queen's death was announced to the public at 6.30pm.