The Queen is under medical supervision at Balmoral after doctors became concerned for her health, Buckingham Palace has said.
Prince Charles, Camilla and Prince William are currently travelling to Balmoral.
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."
Queen Elizabeth II welcomes Liz Truss during an audience at Balmoral, Scotland, where she invited the newly elected leader of the Conservative party to become Prime Minister and form a new government. Picture date: Tuesday September 6, 2022.
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 reign, in a break from tradition.
Immediate family members have been told of the doctor's decision, Sky News' royal correspondent, Rhiannon 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."
Prime Minister Liz 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."
He added: "If there is anything else, we will update the House accordingly."
Leader of the opposition, Sir Keir Starmer tweeted expressing his concern for the monarch.
Other key figures including the Archbishop of Canterbury, and our local MPs, have expressed concern for the Queen.
David Morris, MP for Morecambe and Lunesdale, said: "Concerning news from the Palace this afternoon. My thoughts are with Her Majesty the Queen and her family."
Cat Smith, MP for Lancaster and Fleetwood, said: "The collective thoughts and prayers of the people of Lancaster and Fleetwood, I am certain, are with Her Majesty the Queen and the Royal Family.
"We pray for her health, her comfort and also for the fortitude of her loved ones."
Her Majesty marked a historic 70 years on the throne with Platinum Jubilee celebrations in June.
But she was forced to miss out on a thanksgiving service at St Paul's Cathedral after experiencing discomfort during the birthday parade and flypast one day earlier.
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