Family members rushed to be by the side of Queen Elizabeth after doctors said they were concerned about the health of Britain's 96-year-old monarch on Thursday, saying she should remain under medical supervision.
The queen, Britain's longest-reigning sovereign and the world's oldest monarch, has been suffering from what Buckingham Palace has called "episodic mobility problems" since the end of last year.
"Following further evaluation this morning, The Queen’s doctors are concerned for Her Majesty's health and have recommended she remain under medical supervision," the palace said in a statement.
"The queen remains comfortable and at Balmoral."
Her eldest son and heir Prince Charles and his wife Camilla have travelled to her Scottish home Balmoral Castle, where she is staying, along with his eldest son Prince William, officials said.
"My prayers, and the prayers of people across the @churchofengland and the nation, are with Her Majesty The Queen today," Justin Welby, the Archbishop of Canterbury, said on Twitter.
Last October, Elizabeth spent a night in hospital and she has been forced to cut back on her public engagements since then. On Wednesday she cancelled a virtual meeting with senior ministers after being advised to rest by her doctors.
The previous day she had been pictured appointing Liz Truss as the country's new prime minister at Balmoral, the 15th premier of her record-breaking reign.
A palace source said immediate family members had been informed and played down speculation that the monarch had suffered a fall.
Elizabeth has been queen of Britain and more than a dozen other countries since 1952, including Canada, Australia and New Zealand, and earlier this year marked her 70th year on the throne with four days of national celebrations in June.
"I have been inspired by the kindness, joy and kinship that has been so evident in recent days, and I hope this renewed sense of togetherness will be felt for many years to come," she said at the time.
Truss said the whole country would be deeply concerned by the news.
"My thoughts – and the thoughts of people across our United Kingdom – are with Her Majesty The Queen and her family at this time," she said.
Lindsay Hoyle, the speaker of the House of Commons, interrupted an energy debate in parliament to say he sent his best wishes to the monarch.
"Along with the rest of the country, I am deeply worried by the news from Buckingham Palace this afternoon," opposition Labour leader Keir Starmer said.