Simon Clarke, the Chief Secretary to the Treasury, and Kwasi Kwarteng, the Business Secretary - who both back Ms Truss - have made the allegation in a Telegraph article, see below.
The pair - two of the most senior government ministers with economic briefs - said that Mr Sunak “dug his heels in as Chancellor” on reforms only possible after the UK left the European Union.
They have named two specifics - attempts to ditch the EU’s Solvency II rule, which makes it harder for pension funds to invest in UK infrastructure projects, and the Northern Ireland Protocol Bill, aimed at unilaterally changing the terms of trade in the province.
“He talks about cutting EU regulations yet dug his heels in as Chancellor against efforts to do exactly that and realise the benefits of Brexit. We both saw it in Cabinet,” wrote the pair.
A Sunak campaign source has pushed back, saying it was “categorically wrong” that he had slowed efforts to ditch the Solvency II rule.
The source added that Mr Sunak backed standing up to Brussels over Northern Ireland trading tensions, even while noting a trade war with the EU could have damaging consequences.
The article from Mr Clarke, who worked under Mr Sunak in the Treasury, and Mr Kwarteng also included a string of fierce swipes at the former chancellor, escalating the war of words between two camps.
At one point, they wrote: “Rishi has given up. He wants people to fear there is no alternative to sliding into recession and that the only option is to stem the bleeding with Labour-lite economic policy.”
They also said: “We simply cannot stick with the status quo. Rishi Sunak likes to talk about fairytales, but his biggest fairytale of all is that we must somehow reject true Conservative solutions of tax cuts as a way of solving the challenges we face.”
The pair also said of Mr Sunak: “He says one thing, then does another.”
The comments effectively accuse Mr Sunak of trying to scare the public about the UK’s economic prospects - a blunt accusation delivered on the record by two of the country’s most senior government figures.
On Wednesday, Mr Sunak used a BBC interview to say that he would rather lose the contest than make "false" promises, in a clear swipe at economic claims being voiced by his rival Ms Truss.
The interventions come after days of the Foreign Secretary being under pressure to spell out in more detail how she would support people through the cost of living crisis. It also follows a day of escalating personal attacks.
Ms Truss said in an interview with the Financial Times published on Saturday that she was against “handouts” to people.
The intervention comes after days of Ms Truss being under pressure to spell out in more detail how she would crisis. It also follows a day of escalating personal attacks.
Ms Truss, the Foreign Secretary, said in an interview with the Financial Times published on Saturday that she was against “handouts” to people.
The Sunak campaign jumped on the comment and said it was proof that Ms Truss would not provide any extra spending for families facing soaring energy costs this autumn.
This week, the Truss campaign has increasingly stressed that there would be extra help for those worst affected - even while providing almost no detail about extra spending help.
On Wednesday, Ms Truss doubled down on her emphasis on tax cuts to ease the cost of living, saying: "If the only answer to everything is to whack up taxes and give out more benefits, then the country is going to run into trouble.”
The campaigns also traded swipes over the topic. The Sunak campaign accused Ms Truss of having a “major U-turn” after a prominent MP supporting her said her cost of living plan was “as clear as mud”.
A Truss campaign spokesman later hit back, saying: “Rishi Sunak wouldn’t know how people benefit from a tax cut because he has never cut a tax in his life. People didn’t vote for the Conservative Party to be subjected to old-fashioned Gordon Brown style politics of envy.
“You cannot tax your way to growth and Liz’s agenda is to build a high wage, high growth, low tax economy that supports people.
“Liz believes in people keeping more of their own money, not Rishi’s socialist tax and spend which will lead us to recession.”
A Sunak campaign spokesman then issued another comment, accusing Ms Truss of policy changes that were not “moral”.
“It’s all very well offering empty words about ‘doing all you can’,” said the spokesman. “But there aren’t lots of different ways to act on this. Taking action means providing direct support, which Truss had previously dismissed as ‘handouts’.”
“Twice now, Truss has made a serious moral and political misjudgment on a policy affecting millions of people, after last week reversing plans to cut the pay of teachers and the Armed Forces outside London.
“Mistakes like this in government would cost the Conservative Party the next general election.”
The bruising personal nature of the attacks has led to questions about whether the final two candidates could really serve in the other’s Cabinet, as both Mr Sunak and Ms Truss have said they would if offered a role.
A senior Labour source told The Telegraph that the blue-on-blue attacks amounted to a “scorched earth” policy that was damaging the Conservative Party and benefitting the Labour Party.
Rishi Sunak wants us to fear there is no alternative to recession. We need a new direction
By Simon Clarke, Chief Secretary to the Treasury, and Kwasi Kwarteng, Business Secretary
Our party is at its best when we have the courage of our convictions.
That is why 14 million people elected this government two years ago, some for the very first time – in part because they wanted lower taxes, a more vibrant, job-creating private sector in their local area, and to make good on the huge economic opportunities of Brexit.
In these turbulent times, we need to stand firm in our belief as Conservatives that our first choice should be about helping people keep more of their money and promoting enterprise.
That means pursuing a bold strategy of low taxes and high growth in line with the values of our great party. We don’t need a failed Gordon Brown approach, where taxpayers are expected to be grateful to receive some of their own hard-earned money.
This is at the heart of Liz Truss’s clear plan to turn our economy around. It is vital we help people with the cost of living by cutting taxes now, not at the end of the decade as Rishi Sunak is suggesting.
But just as important is getting our economy going so we can grow the pie – new businesses, more jobs and higher pay to avoid the misery of a recession.
Liz’s plan is all about growth. She will cut taxes and deliver the radical supply-side reforms we need and put the economy on a sound footing.
We both worked in Cabinet with Liz in economic ministries, and we stand with her in our shared determination to tackle the cost of living, keep taxes low and drive through urgent reform - including her commitment to cut business-burdening EU law from our statute books by the end of 2023.
Under her leadership, we will deliver on our promises to the British people, making most of our newfound-freedoms outside the European Union and levelling up our country in a truly Conservative way.
We simply cannot stick with the status quo. Rishi Sunak likes to talk about fairytales, but his biggest fairytale of all is that we must somehow reject true Conservative solutions of tax cuts as a way of solving the challenges we face.
We cannot and must not normalise the highest tax burden in 70 years, as Rishi wants to, and we need a serious plan for economic growth.
He says one thing then does another. That is why our colleague Chris Skidmore MP switched support from Rishi to Liz, saying: “I have grown increasingly concerned by his campaign’s consistently changing position, especially on the economy, to chase votes.”
Rishi has given up. He wants people to fear there is no alternative to sliding into recession and that the only option is to stem the bleeding with Labour-lite economic policy.
This cannot be the sort of country or economy we want to be.
Liz has been consistent in advocating tax cuts and a true Conservative plan on the economy since the start of this campaign.
Rishi, in contrast, has changed position - doing a virtual 360 in refusing to promise any support at the start of this campaign, to then flip-flopping on handouts and VAT. He did one thing as Chancellor and is now promising another as a candidate.
He talks about cutting EU regulations, yet dug his heels in as chancellor against efforts to do exactly that and realise the benefits of Brexit. We both saw it in Cabinet, including resisting reforms to the EU's Solvency II regulation - making it harder for pension funds and investors to invest in British business and infrastructure – and being backward-leaning on moving ahead with legislation to fix issues with the Northern Ireland Protocol.
British families deserve a clear plan and someone who can be trusted to deliver what they have promised. In the 2019 manifesto, we promised not to raise National Insurance.
Reversing Rishi’s National Insurance increase and suspending green levies on energy bills will therefore help struggling families with the cost of living and boost growth.
As you would expect, there is work going across government on what more can be done, and Liz has been clear that she will do all that she can to help those struggling with the cost of living.
She has already committed to keep the existing Energy Bills Support Scheme, which will see a £400 discount paid to consumers over six months from October, and the £1,200 package of support for the most vulnerable.
The responsible thing to do is to consider these proposals when in office, making decisions in light of all the facts and the options. That’s what leadership is about.
Liz has made clear she would hold an emergency Budget and review government spending to see what efficiencies can be found.
It would be highly improper to expect such important policies to be announced during a leadership campaign without sight of all the details of the pressures people could face.
Like the rest of Europe, we are set for tough economic times, with rising costs across the board. At least Liz actually has what it takes to see us through the current turmoil.
She is a tried and tested leader who has proven herself time and time again, whether facing down Vladimir Putin or striking dozens of post-Brexit trade deals.
Britain’s decline is not inevitable. As in 1979, we need a leader prepared to do things differently.
We need a new direction, and her plan offers that. Liz has an optimistic vision for what our economy can become - a high-growth and highly productive powerhouse - and the tenacity to make it happen.
She will provide the confident, clear and courageous leadership Britain needs. She will get our economy growing again and do so by being true to her solemnly-held principles of freedom, enterprise, and aspiration.
Together, we will defy the voices of decline and ensure the United Kingdom’s greatest days lie ahead.