Key to Sir Keir’s chances of establishing stability and competence at the heart of Government is getting the best person in place as the country’s top civil servant when Simon Case departs. The new cabinet secretary could be announced by the end of next month, with Antonia Romeo, permanent secretary at the Ministry of Justice, seen as a leading contender.

He or she will have to rapidly establish an excellent working relationship with Sir Keir’s new chief of staff, Morgan McSweeney, who took on the role following the ousting of Sue Gray. Speculation is rife that Ms Gray will go to the Lords.

Tories say the country has endured “100 days of hard Labour”.

Shortly before the summer election, Mr Sunak delivered a stark warning that Sir Keir would inflict “irreversible damage” on the nation within 100 days. Now – with less than three weeks to go before we learn whether he will be succeeded by Robert Jenrick or Kemi Badenoch – he says we are seeing the “realities” of a Government that puts “country second”.

He warned that if Chancellor Rachel Reeves presses ahead with changes to borrowing rules in this month’s Budget so she can spend more on major projects she will be guilty of “fiddling the figures”.

The former PM is clear that Labour is in danger of wrecking Britain as it hits parents who send their children to independent schools with a tax bomb and delivers inflation-smashing pay deals for workers in militant unions.

Mr Sunak said: “Whether it is cutting the winter fuel cut to pensioners to fund a pay rise for train drivers, scrapping the Conservatives’ plans to secure our borders, or imposing an ideologically-driven tax raid on schools, Labour are making the wrong choices for the country.

“Now Labour are trying to change the rules to let them borrow more money – something the Chancellor herself said would be tantamount to ‘fiddling the figures’. We are all seeing the realities of a Labour Government that puts party first and country second.

“The chaotic first 100 days of this Government show that Labour won’t deliver for Britain and must be held to account for their broken promises.”

Mr Sunak’s intervention triggered outrage in Labour ranks.

A source said: “Labour won’t take lessons from a man who knowingly left a £22billion black hole in the country’s purse for Labour to fix. This is the worst inheritance for a Government in living memory, and this cataclysmic Tory failure means we face tough choices to stabilise the economy.

“Unlike them, we will not play fast and loose with the public’s money. In 100 days, Labour has already made more positive change in government than the Tories made in 14 years.

“And whilst they plan cuts to maternity pay, we continue our work of change at pace.”

But Shadow Chief Secretary to the Treasury Laura Trott denounced Labour for increasing “uncertainty in the economy in the hope of political gain”.

Anxiety about this month’s Budget has mounted ever since the Prime Minister and the Chancellor made gloomy pronouncements about the state of the national finances shortly after arriving in Downing Street.

Ms Reeves announced in July there was a “£22 billion hole” and the following month Sir Keir said that “things will get worse before we get better”.

Consumer confidence fell sharply last month. Business confidence has also taken a knock.

Ms Trott said: “If the Chancellor had triggered the Budget process immediately on getting into office, this week we could have been talking about their plans, ending the uncertainty which is affecting businesses. Instead, she has taken great delight in talking down our economy, and focused her time on trying to set a discredited narrative about a black hole.”

Shadow immigration minister Paul Holmes did not hide his anger at Labour for shredding the Tories’ flagship plan to tackle illegal immigration.

The PM declared the Rwanda scheme “dead and buried” in his first Downing Street press conference. But more than 13,000 people have crossed the Channel this way since Labour too power.

Mr Holmes said: “They have officially opened the doors to people smugglers.”

Alp Mehmet of Migration Watch described Labour’s record as “100 days going backwards,” adding: “The PM and his Home Secretary seem clueless on how to stop the armada of illegal boats.”

There is deep concern about Labour’s plan to slap VAT on school fees.

Shadow Education Secretary Damian Hinds accused the Government of “rushing through” the tax hike “despite having published no official analysis of what it will actually raise, nor the impact on the state sector, or to the military families, or children with special needs”.

This is by no means the only decision that worries him.

“They’ve scrapped the Ofsted summary grade without saying what will replace it,” he said. “They’ve paused the phase-out of low quality technical and vocational qualifications.

“They’ve stopped the commencement of the Free Speech Act.”

Meanwhile, Shadow Energy Secretary Claire Coutinho accused Labour of “economic insanity” when it comes to net zero.

Sir Keir’s party went into the election trumpeting its “green prosperity plan” to “create 650,000 good jobs, cut bills by £300 on average and deliver real energy security”.

But the latest hike in the energy price cap has pushed gas and electricity prices up 10 per cent, hiking the typical dual-fuel bill to £1,717.

Ms Coutinho said: “Let’s be clear, in the general election Labour promised to cut people’s energy bills by £300. But their first act in office has been to take that amount away from pensioners in poverty this winter.

“For 10 million pensioners, Labour’s first 100 days have been a disaster. Ed Miliband won’t stop there, as his reckless net zero plans will see bills go through the roof and jobs sent to China.”

With the Middle East in turmoil and ongoing war in Ukraine, there is anxiety about the lack of a clear timetable to hike defence spending to 2.5 per cent of GDP. The Conservatives had pledged to hit the target by 2030-31.

The National Audit Office last year warned of a multi-billion pound shortfall in its equipment plan.

Shadow Defence Secretary James Cartlidge said: “Lights on the global dashboard are flashing red and Britain is facing real threats to our safety, freedom and security. Despite this, Labour quickly shelved our plan to increase defence spending whilst the world gets more dangerous.

“In just 100 days, Labour have made their political priorities clear. No matter the trade off, our national security comes a distant second every single time.”

Shadow Foreign Secretary Andrew Mitchell described the decision to hand over sovereignty of the Chagos Islands – home to the UK-US Diego Garcia military base – to Mauritius as “quite extraordinary”. Critics were not pacified by Mauritius agreeing a long-term deal for the base to remain for at least 99 years, and there are fears China’s influence will grow in this strategically-important stretch of the Indian Ocean.

Mr Mitchell said: “There seems no rhyme or reason in the government’s thinking.”

Closer to home, there are worries about the power of trade unions over the Government. Most train drivers will enjoy a £9,000 boost after accepting a three-year deal and junior doctors won a 22 per cent pay rise over two years.

In August the Government announced the repeal of a law which allowed “minimum service levels” to be required during strikes in areas including education, fire and rescue, health and border security

Shadow Business and Trade Secretary Kevin Hollinrake said: “When their union paymasters say jump, Labour simply ask, ‘How high?’ The rolling back of checks and balances on union powers means that they can once again hold the country to ransom.

“We all know where this ends. Back in the 1970s.”