The Queen's four children have travelled to Scotland to be by her side after she was placed under medical supervision at Balmoral.
In a rare update on the Monarch's health, a Buckingham Palace spokesman said on Thursday: “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.”
The 96-year-old has ongoing mobility issues and looked bright but frail earlier this week when she appointed Liz Truss as prime minister.
An RAF plane carrying the Duke of Cambridge, the Duke of York and the Earl and Countess of Wessex landed in Aberdeen around 4pm.
They were seen arriving at the castle in a car driven by Prince William at around 5pm.
They joined the Prince of Wales, the Duchess of Cornwall and the Princess Royal at Balmoral.
The Duke of Sussex has also travelled to Balmoral to join the Royal Family at the Queen's side.
Sources have told Yahoo News UK the Duchess of Sussex has stayed behind in London.
This means all four of the Queen's children will be at Balmoral, as well as Prince Charles's two children.
At the time of reporting, the Duchess of Cambridge has remained in Windsor, where her children started their first day of school earlier on Thursday.
Tuesday's meeting with Truss marked the first time in the Queen's 70-year reign she did not appoint the PM at Buckingham Palace.
It was the first time she had been seen in public for 47 days after she travelled to Scotland to enjoy her annual summer break at Balmoral, thought to be one of the Queen's favourite places.
While still smiling, the Queen relied on a walking stick and showed some bruising on the back of her hands.
Her Majesty pulled out of a virtual Privy Council on Wednesday after doctors ordered her to rest.
Truss said on Thursday: “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”.
Labour leader Sir Keir Starmer said he was “deeply worried” by the news from Buckingham Palace and he is hoping for the Queen’s recovery. He 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.”
Scotland’s first minister Nicola Sturgeon said: “All of us are feeling profoundly concerned at reports of Her Majesty’s health. My thoughts and wishes are with the Queen and all of the Royal Family at this time.”
Meanwhile, Welsh first minister Mark Drakeford said in a tweet: “Concerned to hear the news from Buckingham Palace.
“I send my best wishes to Her Majesty and her family on behalf of the people of Wales.”
Justin Welby, the Archbishop of Canterbury, said: "My prayers, and the prayers of people across the (Church of England) and the nation, are with Her Majesty The Queen today."
"May God’s presence strengthen and comfort Her Majesty, her family, and those who are caring for her at Balmoral."
In wake of the news, the BBC suspended regular programming in favour of rolling news coverage.
ITV also announced a special extended version of its evening news programme starting from 5pm.
The Queen has missed a number of high-profile engagements in recent years.
In October last year she secretly spent a night in hospital after undergoing tests, and was ordered by doctors to rest for the next three months.
She subsequently missed the Remembrance Sunday Cenotaph service in London and COP26 climate change talks in Glasgow.
The Queen then tested positive for COVID in February and suffered from mild cold-like symptoms. She said the virus left her "very tired and exhausted".