Prince Charles and Camilla are at the Scottish estate and Prince William is on his way, Clarence House has confirmed.
Her Majesty, 96, postponed her audience with the Privy Council on Wednesday on doctors’ orders.
A day before the monarch swears in new Prime Minister Liz Truss in Scotland, in a break with tradition, amid ongoing mobility problems in her old age.
Speaker Lindsay Hoyle interrupted a House of Commons debate on the energy crisis on Thursday to update MPs on the monarch’s health.
He said: “I know I speak for the whole 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 time.”
The Princess Royal is at Balmoral and the Duke of York and the Earl and Countess of Wessex are on their way to the Queen’s Scottish home, sources said.
Prince Andrew is also on his way to the royal estate.
Liz Truss said: “The whole country will be deeply disturbed by the news from Buckingham Palace this afternoon.
“My thoughts – and the thoughts of the people of our United Kingdom – are with Her Majesty the Queen and her family at this time.”
Labor leader Keir Starmer also said he was “deeply concerned” by the news.
A Buckingham Palace spokesman said: “Following further assessment this morning, the Queen’s doctors are concerned about Her Majesty’s health and have recommended that she remain under medical observation.
“The Queen remains comfortable at Balmoral too.”
The Queen invited Liz Truss to form a government on Tuesday
/ PAThe Archbishop of Canterbury said the “nation’s prayers” were with the Queen.
“My prayers and those of the people of the (Church of England) and the nation are with Her Majesty the Queen today,” Justin Welby tweeted.
“May God’s presence strengthen and comfort Her Majesty, her family and those who care for her at Balmoral.”
The Duchess of Cornwall was due to open a new visitor center for the Great Tapestry of Scotland in Galashiels, in the Scottish Borders, on Friday. This is now likely to be cancelled.
No news yet on Prince Harry’s trip to Balmoral. He is currently in the UK with his wife Meghan.