Liz Truss was informed that the Queen was gravely ill before she entered Parliament to deliver a major statement on energy bills.
It had been believed that the new Prime Minister first became aware of the Queen's condition when she was handed a note in the Commons by Nadhim Zahawi, the Chancellor of the Duchy of Lancaster, shortly after 12pm.
Labour leader Keir Starmer and his deputy Angela Rayner, and Commons Speaker Sir Lindsay Hoyle were also briefed, setting off a flurry of speculation before Buckingham Palace released a statement saying doctors were concerned for the Queen's health.
It has now emerged that Ms Truss was briefed on events before she arrived in the Commons.
No one in No10 had thought much of it when a routine briefing about Operation London Bridge - the preparations for the Queen's death was cancelled on Wednesday afternoon.
“[The briefing] was cancelled because of the energy announcement and the reshuffle,” a No 10 source told the Sunday Times. “Little did we know.”
The new Prime Minister had been prepping for what was expected to be a defining moment of her first days in office when she was interrupted by Cabinet Secretary Simon Case early on Thursday morning, the Mirror understands.
Mr Case, a former private secretary to Prince William, had been notified by the Palace about the Queen's condition and informed the PM, before setting the wheels in motion behind the scenes.
Ms Truss was then in her Commons office, making the final preparations for her statement, when she was informed that the Queen’s death was believed to be “imminent”.
The news was greeted with “shock, immense sadness and incredulity”, said a source told the Telegraph.
Ms Truss arrived to deliver the statement 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.
The note passed by Mr Zahawi to the PM said that Buckingham Palace would shortly issue a statement about the monarch's condition.
The shock in the Commons was palpable when the Palace confirmed that doctors were concerned for the Queen's health, with Sir Lindsay Hoyle giving a brief statement to convey that the thoughts and prayers of Parliament were with the monarch.
At 4.30pm Ms Truss was informed that the Queen - who she had met only two days before as she became PM - had died.
Ms Truss began to prepare her speech, which she is said to have written herself after rejecting a pre-prepared address by the civil service.
The news was made public two hours later, sending shockwaves around the world.
