Queen Elizabeth II is under medical supervision at Balmoral after doctors became concerned for her health, Buckingham Palace officials said.
Prince Charles, Camilla, Prince William are currently travelling to Balmoral, Clarence House and Kensington Palace have said.
Prince Andrew is also understood to be travelling to the Scottish residence, Sky's royal correspondent Rhiannon Mills said.
Buckingham Palace said in a statement: "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."
Her Majesty postponed her Privy Council meeting last night after being advised by doctors to rest.
On Tuesday, the Queen, 96, met outgoing PM Boris Johnson before appointing the new prime minister, Liz Truss, at Balmoral for the first time in her 70-year reign, in a break from tradition.
Immediate family members have been told of the doctor's decision, Mills said.
They are said to be very keen there is no further speculation - amid suggestions the Queen had suffered a fall, which are believed to be incorrect.
Mills said: "Of course, there undoubtedly is going to be concern about the Queen.
"It was only last night at six o'clock that we were informed by Buckingham Palace that the Queen had to pull out of conducting a Privy Council meeting which had been pencilled in yesterday evening.
"Last night, they said that after a full day on Tuesday, Her Majesty had accepted the doctor's advice to rest, which had meant the Privy Council meeting that she was due to be conducting, albeit remotely from Balmoral, wasn't able to go ahead."
The Queen had been due to hold the Privy Council meeting of senior government advisers virtually on Wednesday evening but it was decided it would be rearranged.
Ms Truss said the whole country would be "deeply concerned" by the news, adding her thoughts were with the monarch and her family.
Speaker Sir Lindsay Hoyle interrupted a debate on energy bills in the House of Commons to update MPs about the Queen's health.
He cut short SNP Westminster leader Ian Blackford's speech to say: "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."
Other key figures including the Archbishop of Canterbury, opposition leader Sir Keir Starmer and former PM David Cameron have expressed their concerns for the Queen.
The royal family website appeared to crash after the news of the Queen's health concern emerged.
An error message appeared on screen reading: "Gateway time-out".
Episodic mobility issues
The Queen has faced ongoing "episodic mobility problems" since last autumn and has been relying on a walking stick.
Her Majesty used a walking stick for the first time at a major engagement during a service at Westminster Abbey last October.
She was forced to miss out on some celebrations to mark her Platinum Jubilee in June, including a thanksgiving service at St Paul's Cathedral after experiencing discomfort one day earlier.
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