The Queen is under medical supervision at Balmoral after doctors became concerned for her health, Buckingham Palace said.
“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 Queen remains comfortable and at Balmoral,” a Palace spokesperson said.
The Prince of Wales and the Duchess of Cornwall have travelled to Balmoral, while the Duke of Cambridge, Prince Andrew and Princess Anne are all on their way to the monarch’s Scottish home.
The Duke and Duchess of Sussex, who had been due to attend a charity event in London on Thursday evening, also made the journey north.
Politicians were quick to send their best wishes to the Queen and her family after they learned of Buckingham Palace’s statement.
“I know I speak on behalf of the entire House when I say that we send our best wishes to Her Majesty the Queen and that she and the royal family are in our thoughts and prayers at this moment,” Commons Speaker Sir Lindsay Hoyle said.
Prime minister Liz Truss, who met the Queen at Balmoral on Tuesday as she was confirmed as the country’s new leader, added that “the whole country will be deeply concerned by the news from Buckingham Palace this lunchtime”.
“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.
Labour leader Sir Keir Starmer said he was “deeply worried” by the news, adding that his thoughts were also with the Queen and her relatives.
“I join everyone across the United Kingdom in hoping for her recovery,” he said.
Meanwhile, Scotland's first minister Nicola Sturgeon said “all of us are profoundly concerned at reports of Her Majesty’s health”, while the Archbishop of Canterbury Justin Welby remarked that the “prayers of the nation” were with the Queen.
The royal family's website appeared to crash following the news of the Queen's health concerns.
The monarch, 96, has ongoing mobility issues and had to postpone a virtual Privy Council meeting that had been scheduled for Wednesday.
New prime minister Ms Truss was due to take her oath as First Lord of the Treasury at the meeting and her new cabinet ministers were due to be sworn into their roles.
Doctors did not advise a hospital stay and Queen remains at Balmoral Castle, where she appointed Ms Truss as the 15th prime minister of her reign earlier this week.
Mr Johnson also met the Queen at her Scottish retreat to offer his resignation, three years after he became prime minister.
Her Majesty’s health has come under increasing scrutiny over the past year, since she has had to pull out of key events due to “episodic mobility problems”.
In October 2021, she used a walking stick at a Westminster Abbey service – the first time she had done so at a major engagement.
A week later, after a busy autumn programme, she was ordered to rest by her doctors and was advised to cancel a trip to Northern Ireland.
The Queen was secretly admitted to hospital for “preliminary investigations” and had her first overnight stay in hospital for eight years on 20 October 2021.
For more than three months she carried out only light duties, including virtual and face-to-face audiences in the confines of Windsor Castle.
She waved to crowds from the Buckingham Palace balcony on the first day of her Platinum Jubilee celebrations on 2 June, later appearing at Windsor Castle. But the following day she pulled out of the Platinum Jubilee Service of Thanksgiving at St Paul’s Cathedral after experiencing “discomfort”.
In March, it was reported that the Queen would not return to living at Buckingham Palace but continue permanent residence at Windsor Castle.
