Liz Truss informed early on Thursday that Queen may die that day

Prime minister decided to press ahead with speech unveiling £100bn energy price freeze

Liz Truss already knew the Queen was on the brink of death when she rose to unveil her £100bn energy price plan in the House of Commons on Thursday morning, it has emerged.

The first indication most people had that anything was amiss came when chancellor of the Duchy of Lancaster Nadhim Zahawi showed Ms Truss a note in the Commons chamber at around 12.15, some 15 minutes before the official announcement of the Queen’s ill-health from Buckingham Palace.

But it is now understood that the prime minister was informed by cabinet secretary Simon Case of the grave situation around two hours before beginning her crucial speech at 11.40am.

Mr Case, a former private secretary to Prince William, told Ms Truss at about 9.30am that he had been informed by the Palace that the monarch was gravely ill and it was “matter of hours, not days”, according to a report in the Mail on Sunday which has not been disputed by Downing Street.

“The mood was one of shock, immense sadness and incredulity,” a source told the paper. “We started the day by preparing one of the biggest economic interventions in British history and ended it by marking the accession of a new King.”

The decision was taken to press ahead with the debate on the energy crisis, in which Ms Truss revealed plans to ensure that average annual domestic gas and electricity bills total no more than £2,500 per household over the next two years.

It was the first major intervention of her premiership and a decision which normally would have been expected to dominate newspaper front pages the following day and could form a significant part of her eventual legacy.

The PM – who had met the Queen at Balmoral only two days earlier to be invited to form a government – gave no indication during her speech of the momentous news that she was expecting.

She wore a sober dark green dress but had not yet changed into the black garb she has adopted since the monarch’s death.

Labour leader Sir Keir Starmer was wearing a patterned black tie, but The Independent understands that this was a coincidence and he was not told of the Queen’s illness until he was passed a note as he responded to Ms Truss’s comments.

Both the PM and Sir Keir left the chamber, sparking speculation that a major incident had occurred.

Immediately after the Palace announcement, Commons speaker Sir Lindsay Hoyle broke into proceedings at 12.36am to inform MPs of the situation and to offer parliament’s “thoughts and prayers” to the ailing monarch.

Ms Truss later met advisers in the No 10 flat to plan a speech paying tribute to Her Majesty, which was delivered in Downing Street shortly after the Queen’s death was announced at 6.30pm.