The Queen did not go to church on Sunday after being advised by doctors to rest, the Sun reported.
The monarch, 95, was expected at a Sunday service at All Saints Chapel in Windsor but did not attend.
It comes after the Queen canceled a trip to Northern Ireland and spent a night in hospital a few days ago for preliminary tests, a Buckingham Palace spokesperson said.
She was reportedly disappointed not to be able to travel to Northern Ireland on Wednesday and reluctantly took her doctors’ advice to get more rest.
The Queen is now believed to be saving her energy for the COP26 eco-summit which she is due to attend when it begins next week.
It is the first time Her Majesty has missed prayers at All Saints Chapel in Windsor since returning from her summer vacation.
Royal author Angela Levin told the Sun“I’m glad she didn’t go to church because maybe she’s listening and agreeing to temporarily take a step back.
“Going to church on a Sunday may seem like a small event, but it is a very big thing for the Queen and for any woman her age,” she added. “She is very religious and it is personally very important to her to go to church every Sunday.”
Since the Duke of Edinburgh’s death in April, the Queen has had more than 100 official engagements, with only Prince Charles and Princess Anne having busier schedules in the past seven months.
The Sunday Times reported that the Queen was exhausted as she maintained a busy schedule of private commitments – as well as staying up late to watch TV.
The newspaper said the Queen had ‘a steady stream of lunches and dinners with family and friends’ this month as ‘she doesn’t want to dine alone’.
Following her visit to hospital earlier this week, a Palace spokesperson said: ‘Following medical advice to rest for a few days, the Queen visited hospital on Wednesday afternoon for preliminary enquiries, returning to Windsor Castle at midday today and remaining in high spirits. ”