Queen Elizabeth II cancelled two engagements on Thursday after she tested positive for coronavirus at the weekend, Buckingham Palace said.
"The two virtual audiences that had previously been scheduled to take place today will now be rescheduled for a later date," a spokesman said.
Royal officials announced on Sunday that the 95-year-old head of state had tested positive and had "mild" Covid symptoms, but would continue with "light duties".
Her eldest son and heir, Prince Charles, tested positive for the second time since early 2020 on February 10, two days after meeting his mother at Windsor Castle.
The queen, who is in her record-breaking 70th year on the throne, cancelled similar virtual engagements planned for Tuesday but spoke to Prime Minister Boris Johnson Wednesday.
The monarch and the prime minister hold regular weekly meetings in private. They were conducted by telephone rather than in person due to coronavirus restrictions.
No further engagements are planned for this week but the latest cancellation will inevitably stoke further fears for her health, given her advanced age.
As a precaution, the queen and her husband Prince Philip moved to Windsor, west of London, in March 2020 as the global pandemic hit Britain.
The couple, who were married for 73 years, self-isolated with a reduced number of household staff. Philip, 99, died in April 2021, and his funeral was held under virus curbs.
She returned to official duties after his death and as restrictions were lifted. But she was forced to slow down on medical advice in October last year.
The palace was forced to confirm that she had an overnight stay in hospital after going in for unspecified tests. Since then her appearances have become rarer.
She held a public engagement at her Sandringham residence in eastern England on February 5 on the eve of the anniversary of her accession to the throne in 1952.
- Jubilee tours -
Preparations are well underway to mark the queen's Platinum Jubilee, with four days of public parades, pageants and parties planned for early June.
It will be hoped they will lighten the mood after a disastrous start to the historic landmark.
Her second son Prince Andrew, 62, earlier this month settled a US civil case for sexual assault, after years of scandal at his friendship with convicted sex offenders Jeffrey Epstein and Ghislaine Maxwell.
Public outrage on both sides of the Atlantic saw the queen strip Andrew of his honorary royal titles and charitable positions, in a move designed to protect the institution from further reputational damage.
Charles himself has also come under scrutiny after police in London announced a probe into "cash for honours" claims connected to one of his charities.
Meanwhile his son Prince Harry, in self-exile in the United States with his wife Meghan and their two children, has made waves with another legal case against a British newspaper.
On Wednesday, he filed a complaint for libel against Associated Newspapers over a report about a separate court case against Britain's interior ministry and the funding of his private security.
But the royals were keen to give the impression of business as usual, announcing a series of royal tours to eight of the 14 Commonwealth countries outside Britain where she is also queen and head of state.
Charles and his second wife, Camilla, will visit Ireland from March 23-25, while his eldest son, Prince William and his wife, Catherine, will travel to the Caribbean.
They will be in the Commonwealth countries of Belize, Jamaica and The Bahamas from March 19-26, with the visit likely to be keenly watched after Barbados ditched the queen as head of state in November last year.
The queen's youngest son, Prince Edward, and his wife Sophie, will visit Antigua and Barbuda, Grenada, St Lucia, and St Vincent and the Grenadines from April 22-28.
And her only daughter, Princess Anne, will be in Papua New Guinea, from April 11-13.
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