The Queen has tested positive for Covid, Buckingham Palace has said.
The monarch is experiencing "mild cold-like symptoms" but expects to continue "light duties" at Windsor over the coming week, the palace said.
"She will continue to receive medical attention and will follow all the appropriate guidelines," it added in a statement.
The Queen, 95, had been in contact with her eldest son and heir, the Prince of Wales, who tested positive last week.
It is understood a number of people have tested positive at Windsor Castle, where the Queen resides.
The announcement comes weeks after the Queen became the UK's longest reigning monarch, reaching her Platinum Jubilee of 70 years on 6 February.
She carried out her first major public engagement for more than three months on the eve of her Jubilee, meeting charity workers at Sandringham House.
The Queen had her first vaccine in January 2021 and is believed to have had all her follow-up jabs after that.
BBC royal correspondent Nicholas Witchell said had been taking life "rather more easily" since spending a night in hospital for medical checks in October last year.
On Tuesday, the Queen attended her first official engagement since coming into contact with Prince Charles, holding a virtual meeting with two new ambassadors to the UK.
Prince Charles's wife, the Duchess of Cornwall, tested positive for the virus last weeks, days after her husband.
It was the first time the duchess had caught Covid, and the second time for Prince Charles.
The Royal Household has its own physicians, and the Queen's is Sir Huw Thomas, a consultant at St Mary's Hospital in London and professor of gastrointestinal genetics at Imperial College London.
He is "head of the medical household", which is part of the Royal Household looking after the health of the family.
