Queen Elizabeth II tested positive on Sunday for Covid-19 but aides said her symptoms were "mild", as politicians wished Britain's longest-serving monarch a rapid recovery in her 70th year on the throne.
In what is meant to be a banner year of Platinum Jubilee celebrations, the news comes at a stressful time for the 95-year-old queen with scandals stalking her two eldest sons, Charles and Andrew.
The queen's heir Prince Charles, 73, tested positive for a second time for the coronavirus on February 10, two days after meeting his mother at Windsor Castle. No information was given then on whether she had taken any Covid tests herself.
The queen resumed in-person audiences at the castle last week, but complained to one attendee of suffering from stiffness and was photographed holding a walking stick.
"Buckingham Palace confirm that the queen has today tested positive for Covid," a statement from the palace said.
"Her Majesty is experiencing mild cold-like symptoms but expects to continue light duties at Windsor over the coming week," it said. "She will continue to receive medical attention and will follow all the appropriate guidelines."
While normally secretive about the queen's health, the palace has previously confirmed she is triple-vaccinated against Covid-19.
Britain's Press Association said "it is understood a number of cases have also been diagnosed among the Windsor Castle team".
The news also comes in a week when Prime Minister Boris Johnson is expected to declare a victory of sorts over the pandemic, by announcing the scrapping of remaining legal restrictions in England with the Omicron wave apparently under control.
"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," Johnson tweeted, as members of his cabinet sent their own best wishes.
Keir Starmer, leader of the main opposition Labour party, tweeted: "Get well soon, Ma'am."
Joe Little, managing editor of Majesty magazine, said there was naturally "cause for concern" with the queen two months away from turning 96.
But he said: "In the coming days a very close eye will be kept on her and the indications are that, all being well, it's nothing more than a minor inconvenience."
Nationwide celebrations to mark the queen's Platinum Jubilee are due to be held in June, after she marked 70 years on the throne on February 6.
On the eve of the anniversary, the monarch held a reception for locals at Sandringham, her estate in eastern England.
It was reportedly her largest in-person public engagement since an unexplained health issue saw her spend a night in hospital last October.
The Covid scare comes with the royal family mired in difficulties, including tensions with Charles's second son Prince Harry, who now lives in California with his wife Meghan.
Prince Andrew settled a sexual assault civil lawsuit in the United States last week, reportedly for £12 million ($16.3 million, €14.3 million) -- which newspapers claim she will partly fund.
Meanwhile police in London have said they are investigating claims that a Saudi tycoon was offered UK honours in return for donations to Charles's charitable foundation.
The queen, whose husband Prince Philip died aged 99 last April, has spent much of the coronavirus pandemic at Windsor Castle, with a reduced number of household staff dubbed "HMS Bubble".
Respecting the government's rules on Covid distancing at the time, she sat alone at Philip's funeral, while Johnson and his staff are under police investigation for apparent breaches of the rules during lockdown parties in Downing Street.
The timing of the government's plan to lift pandemic legislation this week could be affected by the queen's illness, according to former minister Caroline Nokes.
"I think it makes it a little bit tricky for them, yes," she told Times Radio.
But speaking on Sky News, royal commentator Alastair Bruce said the queen "would not want anyone to change any decisions on the basis of her state of health".
"I think for a very feisty and determined lady of her mid-90s, she is more than ready to deal with what she faces," he added.
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