A number of other members of the Royal Family have also tested positive for coronavirus – in addition to the Queen.
Her Majesty, 95, is experiencing mild cold-like symptoms and expects to continue light duties at Windsor this week, Buckingham Palace said.
She is understood to be triple vaccinated.
COVID news live: Messages of support pour in for Queen
Prince Charles, 72, emerged from isolation on Thursday after contracting COVID-19 for the second time.
The heir to the throne first got the virus in March 2020 – early in the pandemic – and lost his sense of taste and smell.
He isolated at Birkhall on the Balmoral estate in Aberdeenshire.
His wife, the Duchess of Cornwall, tested positive for the first time last week.
Both Camilla, 74, and Charles are understood to have received booster vaccines.
She had previously described herself as “luckily” negative, saying she had been tested “so many times”.
Duke of Cambridge hit ‘pretty hard’ by COVID
Prince William, 39, contracted the virus in April 2020 but did not make it public.
Revealing the news seven months later, The Sun newspaper said he had not wanted to alarm the nation.
The duke was hit “pretty hard” by the virus and at one stage struggled to breathe, the newspaper added.
Royal doctors treated him as he isolated in Norfolk and he apparently continued with telephone and online meetings.
“There were important things going on and I didn’t want to worry anyone,” William told someone at an engagement.
Princess Anne missed Christmas Day with Queen
Princess Anne’s husband, Admiral Sir Timothy Laurence, tested positive in December last year, meaning they were unable to join the Queen on Christmas Day at Windsor.
Princess Michael of Kent – married to the Queen’s cousin, Prince Michael of Kent – suffered severe symptoms after getting COVID in November 2020.
The 77-year-old is understood to have had lung problems as a child, making her more susceptible to the disease.
And, away from the UK, royals Queen Margrethe of Denmark, 82, and Spain’s King Felipe VI, 54, both tested positive for the illness on 9 February and had mild symptoms.