Liz Truss was passed a note about the Queen's health in the Commons, looking concerned as she read it on only her third day as Prime Minister.
Nadhim Zahawi, who also stood in the recent Tory leadership election, passed a note to the new PM in the House of Commons that laid bare the gravity of the current situation.
MPs have rushed to share their wellwishes for the monarch, 96, who is in Balmoral Castle. A Buckingham Palace spokesperson said the Queen is " under medical supervision ".
A Palace statement 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."
Members of the Royal Family, including the Duke of Cambridge and the Sussexes, have since traveled to the estate in Aberdeenshire to be with the Queen.
Follow the latest updates on the Queen's health today in our live blog
Ms Truss met Her Majesty on Tuesday at Balmoral where she was invited to form a new government having replaced outgoing PM Boris Johnson.
Mr Zahawi, who is the Chancellor of the Duchy of Lancaster, the portfolio of land, property and assets held by the Queen, entered the chamber earlier today as a debate over energy prices continued, and took a seat next to Ms Truss.
The PM looked concerned as she read the note.
It appeared Labour leader Sir Keir Starmer was also notified of the developing situation by his deputy, Angela Rayner.
It was then evident that the sombre message and tone spread through the chamber as those present bore looks of concern and worry.
Since receiving the news, Ms Trust 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."
Meanwhile Sir Keir Starmer added: "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."
There are no plans for the Prime Minister to travel to Scotland on Thursday or Friday, Downing Street said.
The Prime Minister’s official spokesman also said he could not give any updates on the Queen’s health or whether Liz Truss has been in touch with the Palace.
“The Prime Minister is working from Downing Street this afternoon in a series of meetings,” he said.
“As is standard, I wouldn’t get into any contact certainly between the Prime Minister and the Palace.”
Asked if there were any plans for the PM to travel to Scotland on Thursday or Friday, he said: “No.”
Downing Street also said it is not aware of any changes to business in the House of Commons on Friday.
The Prime Minister’s official spokesman said this would be a matter for the Speaker.
Asked if the plan is still to go ahead with the business of the House on Friday, he said: “That’s a decision for the Speaker, fundamentally, but I’m not aware of any change.”
Meanwhile, more than 100 umbrella-clad observers have congregated on the stone steps up to The Queen Victoria Memorial, opposite the royal residence, and dozens more are standing beside the gates.
In Aberdeen, an RAF plane carrying the Duke of Cambridge, the Duke of York and the Earl and Countess of Wessex recently landed in Aberdeen.
It is understood the Duke and Duchess of Sussex are on their way to Balmoral separately from other royals, but have been in co-ordination with other family members’ plans.
