Buckingham Palace released a statement this afternoon saying the 96-year-old monarch was under medical supervision after concern from doctors. The Prime Minister was told of the situation while she was still in the House of Commons answering questions on her energy strategy.
Senior minister Nadhim Zahawi entered the Chamber to inform Ms Truss on the situation, with a series of frenzied whispered conversations then taking place between party leaders and Speaker Lindsay Hoyle.
Both Ms Truss and Labour leader Sir Keir Starmer swiftly left the Chamber shortly afterwards.
In a statement, the Prime Minister 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.”
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In a message of his own, Labour leader Sir Keir Starmer said he was “deeply worried” by the development.
He said: “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.”
Speaker Sir Lindsay Hoyle interrupted the speech of SNP Westminster leader Ian Blackford to announce the update to MPs.
He said: “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.”
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It has been more than 150 years since a monarch last appointed a Prime Minister while in Scotland.
The messages from Ms Truss, Sir Keir Starmer and Sir Lindsay Hoyle were echoed by politicians from across the country.
Scottish First Minister Nicola Sturgeon said she was “feeling profoundly concerned” by the reports.
“My thoughts and wishes are with the Queen and all of the royal family at this time,” she added.
Wales’ First Minister Mark Drakeford said: “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.”
London Mayor Sadiq Khan added: “I’m sure I speak for all Londoners when I express my concern for Her Majesty the Queen following news from Balmoral of her health.
“My thoughts and best wishes for a swift recovery are with Her Majesty and the whole Royal Family at this time.”