- Queen Elizabeth "has reluctantly accepted medical advice to rest for the next few days".
- The 95-year-old monarch cancelled a planned trip to Northern Ireland on medical grounds.
- The palace said she was "in good spirits and is disappointed that she will no longer be able to visit Northern Ireland".
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 not related to the 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 palace said she was "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.
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 sends her warmest good wishes to the people of Northern Ireland, and looks forward to visiting in the future," the statement added.
The queen - Britain's longest-serving monarch - succeeded her father, king George VI, in 1952 and next year celebrates her Platinum Jubilee to mark 70 years on the throne.
She 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.
Last week, she delivered a speech at the opening of the Welsh Assembly in Cardiff, and at the weekend spent a day at Ascot racecourse.
On Monday, she held a virtual audience with the new governor-general of New Zealand, and on Tuesday received two ambassadors, also by videolink.
On Tuesday evening, she hosted a reception at Windsor for international business leaders attending a government investment summit, including the billionaire philanthropist Bill Gates, and senior British ministers.
At Tuesday's reception, the Queen, her eldest son and heir Prince Charles, and grandson Prince William, mingled with guests, none of whom were wearing facemasks.
Coronavirus restrictions were lifted in July but an increase in cases has prompted calls for measures to be reimposed to prevent further close-contact transmission.
England's chief medical officer, Chris Whitty, said masks should be worn in crowded indoor spaces, due to fears that high numbers of cases could put pressure on health services.
The latest palace statement will inevitably raise fears for her health, even though she appeared well 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.
But also on Tuesday, it was 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.