Britain's Queen Elizabeth tested positive for Covid-19 on Sunday, Buckingham Palace said.
The 95-year-old monarch is experiencing mild, cold-like symptoms but expects to continue light duties at Windsor this week, a palace statement said.
She will continue to receive medical attention and will follow all appropriate guidelines, it said.
"I’m sure I speak for everyone in wishing Her Majesty The Queen a swift recovery from Covid and a rapid return to vibrant good health," said UK Prime Minister Boris Johnson.
The queen had been isolating after her eldest son, Prince Charles, was diagnosed with Covid-19 on February 10, only days after a meeting with her.
She resumed public duties on Tuesday, when she received the credentials of the ambassadors of Spain and Estonia via video link from Buckingham Palace.
Camilla, Duchess of Cornwall and wife of Prince Charles, tested positive a few days after her husband.
The prince, 73, had previously contracted Covid-19 in March 2020, in the early stages of the pandemic, but was reported to have suffered only mild symptoms.
The health of the queen, the world's oldest and longest-reigning monarch, has been in the spotlight since she spent a night in hospital in October last year for an unspecified ailment and was then advised by her doctors to rest.