Queen Elizabeth II has cancelled a planned trip to Northern Ireland on medical grounds, Buckingham Palace said on Wednesday.
Royal officials said in a statement that the 95-year-old monarch "has reluctantly accepted medical advice to rest for the next few days".
Britain's domestic Press Association news agency said the decision was understood not to be related to coronavirus and that she was resting at Windsor Castle, west of London.
She was also expected to attend events at the upcoming UN climate change summit in Glasgow next month, it added.
The Queen -- Britain's longest-serving monarch -- moved to Windsor in March last year as the coronavirus outbreak took hold.
The decision was made to self-isolate because of the increased risk of infection due to her age, although she has since been vaccinated.
She has resumed public engagements since the funeral of her late husband, Prince Philip, after his death aged 99 in April, either alone or accompanied by other senior royals.
On Tuesday, she hosted a reception at Windsor for international business leaders attending a government investment summit and received two ambassadors by videolink.
It was also revealed that she had turned down an award celebrating the achievements of older people, assessing that she did not fit the criteria.
"Her Majesty believes you are as old as you feel," her assistant private secretary replied to The Oldie Magazine, it reported on Tuesday.
The latest palace statement will inevitably raise fears for her health, even though she appeared in good spirits at Tuesday's events.
She was seen for the first time at a major public event using a walking stick, although royal officials said it was not linked to any health condition.
"Her Majesty is in good spirits and is disappointed that she will no longer be able to visit Northern Ireland, where she had been due to undertake a series of engagements" on Thursday and Friday, the palace said.
"The Queen sends her warmest good wishes to the people of Northern Ireland, and looks forward to visiting in the future."
A church service to mark the 100th anniversary of the creation of Northern Ireland is due to take place in border town of Armagh on Thursday.
The queen succeeded her father, king George VI, in 1952 and next year celebrates her Platinum Jubilee to mark 70 years on the throne.
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