Liz Truss was told of the Queen's condition before she delivered a speech on Thursday on freezing energy prices for two years.
Footage emerged of the Chancellor of the Duchy of Lancaster Nadhim Zahawi informing the Prime Minister shortly after noon that the monarch was gravely ill.
It has now emerged Truss was briefed on events before she arrived in the House of Commons to deliver the speech which will save the typical household £1000-a-year.
Labour leader Keir Starmer, his deputy Angela Rayner and Speaker Lindsay Hoyle were also told about the Queen's condition in the Chamber which set off a flurry of speculation before Hoyle announced the news.
Officials in Downing Street didn't think much of it when a routine briefing about Operation London Bridge, the preparations for the Queen's death, was cancelled on Wednesday afternoon.
A source told the Sunday Times: "[The briefing] was cancelled because of the energy announcement and the reshuffle. Little did we know."
Cabinet Secretary Simon Case, a former private secretary to Prince William, was informed by Buckingham Palace about the Queen's condition and told Truss on Thursday morning before she headed to the Commons.
Truss arrived in the Chamber with a sombre expression, aware already that her announcement of the largest fiscal intervention since the Second World War was about to be dwarfed by the events to come.
It's believed the PM was in her Commons office when she was informed that the Queen’s death was believed to be “imminent”.
The news of the Queen's condition was eventually announced by Hoyle which sent shockwaves around the Chamber and across the country. Truss was told at 4.30pm that Her Majesty had passed away at Balmoral Castle in Aberdeenshire.
Buckingham Palace released a statement two hours later saying the Queen had passed away.
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