Prince Charles and Camilla have arrived at Balmoral and Prince William is travelling there
The Duke of York - Prince Andrew - is on his way to Balmoral. The Count and Countess of Wessex, Edward and Sophie, are also travelling there, and Princess Anne is already in Scotland where she had been undertaking engagements.
All the Queen's children are either at or travelling to Balmoral.
A Palace spokesperson 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.”
Senior royals are believed to be on their way to Balmoral.
Prime Minister Liz Truss said: “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.”
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.”
Speaker Sir Lindsay Hoyle interrupted the speech of SNP Westminster leader Ian Blackford during the energy debate to tell MPs: “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.”
He added: “If there is anything else, we will update the House accordingly.”
Queen Elizabeth II: Senior Royals on way to Balmoral Castle as Queen remains under medical supervision
Alison Johnstone, Presiding Officer of the Scottish Parliament, said: “I am aware...of the health of Her Majesty the Queen. I will of course monitor developments and keep members updated over the course of the day. But I assure the thoughts of all of Parliament are with Her Majest at this time.”
Labour leader Sir Keir Starmer said he is “deeply worried” by the news from Buckingham Palace and he is hoping for the Queen’s recovery.
He tweeted: “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.”
The royal family’s website appeared to crash following the news of the Queen’s health concerns.
An error message appeared on screen reading: “Gateway time-out”.
Wales’ 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.”
The nation’s longest-reigning monarch has ongoing mobility issues and looked bright but frail and used a walking stick during Tuesday’s historic audience with Ms Truss, which followed a visit from outgoing prime minister Boris Johnson as he tendered his resignation.