The UK Government could have to find more than £50billion in next month's budget to keep the economy on an even keel.
Chancellor Jeremy Hunt and Prime Minister Rishi Sunak are facing "sober" decisions on potential spending cuts and tax rises, officials have told the BBC.
Economic growth is forecast to be considerably lower than the last independent forecast.
The budget announcement on the plans has been pushed back by more than two weeks to Thursday, November 17.
The Treasury would not put a figure on how much the chancellor and prime minister will need to find, but the BBC is told it may need to be at least £50billion.
Multiple sources said the amount of money the government needed to raise through spending cuts and tax rises was also more than just this current "hole" in its finances.
Headroom needed
This, they said, was because the government needed some "headroom" beyond just getting their existing debt down in case the economy does not grow as much as expected.
While government borrowing costs have fallen somewhat in recent days, officials warned Mr Hunt and Mr Sunak that they remain considerably elevated as global interest rates also rise.
The PM and chancellor agreed that, to ensure the package is credible to financial markets, there must also be a buffer - which means a bigger "repair job" than previously expected.
A Treasury source said: "Markets have calmed somewhat, but the picture is still bleak. Britain is facing an economic crisis with a massive fiscal black hole to fill.
"People should not underestimate the scale of this challenge, or how tough the decisions will have to be. We've seen what happens when governments ignore this reality."
Last month, sterling fell to a record low against the dollar as government borrowing costs rose in the aftermath of then-chancellor Kwasi Kwarteng's mini-budget.
The financial markets plunged into turmoil when Mr Kwarteng announced major tax cuts without detailing how they would be paid for.
- Labour's deputy leader Angela Rayner has admitted she is relaxed about people becoming "filthy rich" - as long as they pay their taxes.
She said Labour wants people to create wealth, echoing famous remarks made by the party's former spin doctor Peter Mandelson.
Ms Rayner was questioned about her views on wealth and class.
Asked if Labour believed in aspiration, she said: "Absolutely, 100%."
Her comments represent a shift away from the language used by Labour MPs and shadow ministers under the leadership of Jeremy Corbyn.
When he launched Labour's campaign ahead of the general election in 2019, Mr Corbyn promised to "pull down a corrupt system" and make the UK's wealthy elite pay their way with new taxes.
Pragmatic tone
But speaking to the BBC's Nick Robinson, Ms Rayner - who rose to Labour's front bench when Mr Corbyn was leader - struck a more pragmatic tone.
She was asked if she agreed with Lord Mandelson who, in 1998, said of New Labour: "We are intensely relaxed about people getting filthy rich as long as they pay their taxes."
"If that means that they're going to be filthy rich as he (Lord Mandelson) says, I wouldn't use that term, but people are going to have great wealth," Ms Rayner said.
"It's about sharing that wealth and understanding where that wealth comes from."
Ms Rayner said she would not be attacking the PM for being rich.
Mr Sunak - whose wife, Akshata Murty, is the daughter of an Indian billionaire - is thought to be one of the richest MPs in Parliament.