Deputy Labour leader Angela Rayner has revealed the mystery note she was passed in the Commons on 8 September told her the Queen was unwell, and she had to discreetly persuade her boss to immediately leave the chamber.
Speaking on the News Agents podcast, Ms Rayner said when Nadhim Zahawi handed it to her, and she read its contents, she instantly "read between the lines".
Labour leader Sir Keir had been giving his response to the government's statement on the cost of living crisis when Mr Zahawi, chancellor of the duchy of Lancaster, whispered into the prime minister's ear and handed her a folded piece of paper.
A similar note was passed to Ms Rayner.
Podcast presenter Jon Sopel said: "You could just feel something had happened."
And the deputy leader has now revealed what that was.
She said: "The note was that the Queen is unwell and Keir needs to leave the Chamber as soon as possible to be briefed.
"I read between the lines on that, because you don't get a note saying the Queen's unwell if she has a bit of a cough or cold."
'I felt the gravity of it'
Fellow presenter Emily Maitlis asked her: "What was going through your head?"
The MP for Ashton-under-Lyne went on: "Before that news, it (the cost of living debate) was the most important news of the day, so to get a note... saying he needs to leave the chamber....I felt the gravity of it."
She said in that instant, on a personal level, she felt "incredibly sombre" but that quickly switched to thinking of a way to get the news to Sir Keir without disturbing him too much from what was an "incredibly important" televised speech to Parliament.
"I was trying to think how to get this note to him..... if someone is trying to give you information when you're in the middle of speaking, it's the most distracting thing, so I was waiting for the opportunity to do it.
"I was kind of waiting for my opportunity, and then I looked over at Mr Speaker, (Sir Lindsay Hoyle, the Commons Speaker) and he was giving me the nod to say 'this is actually quite urgent,' ie. don't wait long."
'It changed everything'
After they left the Chamber, Cabinet Secretary Simon Case explained to both of them what was happening.
At 12.20pm, Buckingham Palace announced the Queen's doctors were concerned for her health, with senior members of the royal family travelling to Balmoral Castle in Scotland to be with her.
At 6:30pm that day, the Palace announced she had died.
Ms Rayner added: "We had all known the Queen was getting very frail and there had been conversations about what would happen ............but it was still such a shock - it changed everything.
"My family are big royalists and my children's names are James and Charles," she added.