Queen Elizabeth II is under doctors' supervision, they are concerned about her health

This was announced by Buckingham Palace.

"Following further assessment this morning, the Queen's doctors are concerned about Her Majesty's health and have advised her to remain under medical supervision," the statement said. 

Doctors advised her to be under constant medical supervision. Her Majesty is now in Scotland at Balmoral. Princes Charles and William have already left there.

Part of the royal family has already arrived for Elizabeth II. The Duke of York, the Earl and Countess of Wessex are traveling to the Scottish estate, about 40 miles (64 km) west of Aberdeen. The Duke of Cambridge is also on the road, while the Duchess of Cambridge stayed in Windsor with her children for their first full day at school. Also going to Balmoral are the Duke and Duchess of Sussex, who were supposed to attend a charity event in London.

The UK Prime Minister reacted to the news about the Queen's health.

"The whole country will be deeply concerned by the news from Buckingham Palace this afternoon. My thoughts and those of people across our United Kingdom are with Her Majesty The Queen and her family at this time," she tweeted.

It is known that the Queen postponed a meeting of the Privy Council the day before after doctors advised her to rest.

The BBC notes that there are apparently acute concerns about the Queen's health - much more so than before, as the Queen canceled at the last minute what was supposed to be a virtual meeting of the Privy Council - senior ministers.