The Queen missed a church service on Sunday and is just a week away from being due to travel to Scotland to host a major reception for world leaders at the Cop26 climate change summit.
The 95-year-old monarch is following doctors' orders to rest after undergoing preliminary tests in hospital last week.
It has not yet been confirmed whether the head of state will definitely carry out the high-profile engagement in Glasgow on Monday November 1, but it remains listed as a future engagement on the royal family's official website.
The Sun reported that Sunday's service was the first time she had missed prayers at Windsor's All Saints Chapel since she returned from her summer break at Balmoral.
She was admitted to King Edward VII's Hospital on Wednesday for her first overnight stay in a medical setting in eight years.
The Queen was said to be "knackered" due to a busy social life and her preference for late-night television, as well as a hectic schedule of engagements throughout October, royal sources told the Sunday Times.
Buckingham Palace kept the Queen's stay in hospital a secret, saying on Wednesday she was resting at Windsor and in good spirits after pulling out of a Northern Ireland visit.
Aides issued a statement saying she had undergone "preliminary investigations" at the private clinic after The Sun revealed the hospital stay.
The monarch, who is head of the Church of England, is a devout Christian and regularly attends a service each week, although during the Covid crisis she opted for private prayers with a chaplain.
She is likely to have worshipped this Sunday by receiving private prayers in the tiny private chapel inside Windsor Castle, where the Duke and Duchess of Sussex's son Archie was christened in 2019.
Buckingham Palace has declined to comment.
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