Queen Elizabeth tested positive for COVID-19 on Sunday and is experiencing mild, cold-like symptoms, Buckingham Palace said, adding that the 95-year-old monarch would carry on working.
The palace said the queen would continue with "light" duties at Windsor Castle over the coming week.
"She will continue to receive medical attention and will follow all the appropriate guidelines," the palace said in a statement.
People in the U.K. who test positive for COVID-19 are required to self-isolate for at least five days, though the British government says it plans to lift that requirement for England in the coming week.
The queen has received three doses of coronavirus vaccine.
Both her eldest son Prince Charles, 73, and 74-year-old daughter-in-law Camilla, Duchess of Cornwall have also recently contracted COVID-19. Charles has since returned to work.
Senior politicians sent get-well messages, with Health Secretary Sajid Javid tweeting "Wishing Her Majesty The Queen a quick recovery." Opposition Labour Party leader Keir Starmer wished the queen "good health and a speedy recovery. Get well soon, Ma'am."
Britain's longest-reigning monarch, the queen reached the milestone of 70 years on the throne on Feb. 6, the anniversary of the death in 1952 of her father King George VI.
A fixture in the life of the nation, Elizabeth has been in robust health for the majority of her reign. In the past year she has been seen using a walking stick, and in October she spent a night in a London hospital for unspecified tests.
The Queen's doctors ordered her to rest, and she was forced to cancel appearances at several key events, including Remembrance Sunday services and the COP26 climate conference in Glasgow, Scotland, in November.
This month she returned to public duties and has held audiences both virtually and in person with diplomats, politicians and senior military officers.
The queen has a busy schedule over the next few months of her Platinum Jubilee year, and is scheduled to attend in-person public engagements in the coming weeks, including a diplomatic reception at Windsor on March 2 and the Commonwealth Service at Westminster Abbey on March 14.
On March 29, she is due to attend a remembrance service at Westminster Abbey for her husband Prince Philip, who died in April 2021 at the age of 99.
Public celebrations of the Platinum Jubilee are scheduled for June, with festivities including a military parade, a day of horse-racing and neighbourhood parties over a June 2-5 long weekend.
The Queen is the latest monarch from around the world to catch COVID-19. Queen Margrethe of Denmark, 82, and Spain's King Felipe VI, 54, both tested positive for the illness earlier in February and had mild symptoms.
