The Queen's doctors say they are "concerned" for her health and that she is under medical supervision at Balmoral Castle in Scotland.
"Following further evaluation this morning, The Queen's doctors are concerned for Her Majesty's health and have recommended she remain under medical supervision," Buckingham Palace said in a statement.
"The Queen remains comfortable at Balmoral."
Prince Charles and his wife Camilla, Duchess of Cornwall are at Balmoral with the Queen and her only daughter Princess Anne, the Princess Royal.
Later on Thursday afternoon Prince William, Prince Andrew, Prince Edward and Sophie the Countess of Wessex arrived at Balmoral after flying into Aberdeen.
It is also understood that Prince Harry, who is in the UK for non-royal engagements, is also on his way to Scotland.
British Prime Minister Liz Truss said her thoughts were with the Queen and her family.
"The whole country will be deeply concerned by the news from Buckingham Palace this lunchtime," she tweeted.
"My thoughts — and the thoughts of people across our United Kingdom — are with Her Majesty The Queen and her family at this time."
Opposition leader Sir Keir Starmer said: "Along with the rest of the country, I am deeply worried by the news from Buckingham Palace this afternoon.
"My thoughts are with Her Majesty The Queen and her family at this time, and I join everyone across the United Kingdom in hoping for her recovery."
Governor-General David Hurley, the Queen's representative in Australia, also released a statement.
"Following news from Buckingham Palace, and on behalf of all Australians, the Governor-General and Mrs Hurley's thoughts and prayers are with Her Majesty The Queen and the Royal family at this time," it read.
The Archbishop of Canterbury, Justin Welby, also sent his best wishes to the monarch.
"My prayers, and the prayers of people across the Church of England and the nation, are with Her Majesty The Queen today," he tweeted.
"May God's presence strengthen and comfort Her Majesty, her family, and those who are caring for her at Balmoral."
The Queen's declining health
The announcement comes a day after the Queen, who is 96, was forced to cancel a virtual Privy Council meeting after doctors advised her to rest.
On Tuesday she was photographed for the first time in two months when she appointed Ms Truss as Britain's new Prime Minister inside the Drawing Room at Balmoral.
The monarch has been suffering from what Buckingham Palace has called "episodic mobility problems" since the end of last year.
She has reduced her public appearances in recent months and missed last weekend's Highland Games, a usual highlight of her busy calendar.
In October last year she spent a night in hospital after cancelling a trip to Northern Ireland, with Buckingham Palace saying at the time it had been for "preliminary investigations".
A royal source cited by Reuters said the Queen had stayed in hospital for practical reasons and that her medical team had taken a cautious approach.
She returned to Windsor Castle the following day, where she spent the majority of the COVID-19 pandemic, and she returned to light duties.
That hospital visit saw the start of the Queen scaling back her duties and appearances, with other members of the royal family often standing in for her.
In February she was diagnosed with COVID-19 and experienced mild, cold-like symptoms but again returned to light duties and held virtual audiences soon after recovering.
After skipping the opening of parliament in March, she made a surprise appearance a week later to open the Elizabeth Line, a commuter rail service in London named in her honour.
Walking with a cane, she spent 10 minutes speaking with station staff before leaving in an elevator.
And in May she toured the annual Chelsea Garden Show in London using a golf buggy so she could inspect the gardens and exhibits more easily.
Platinum Jubilee celebrated
This year the Queen became the first British Monarch to celebrate a Platinum Jubilee to mark 70 years on the throne.
The occasion was marked by a series of events across Britain and around the world, but she was not able to take part in most of the festivities.
She made an appearance on the balcony of Buckingham Palace alongside other senior working royal family members for Trooping the Colour the mark the start of the main four-day event in London, as well as a brief appearance on the final day to mark the Jubilee pageant.
The Queen said she was "deeply humbled and deeply touched" with the celebrations.
"While I may not have attended every event in person, my heart has been with you all; and I remain committed to serving you in the best on my ability, supported by my family," she said in a statement.
"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."
In 2021 the Queen's husband of 73 years, Prince Philip, the Duke of Edinburgh, died at age 99.
