Liz Truss has doubled down on her reluctance to “bung more money” at those who will struggle to afford spiralling energy costs this winter while Rishi Sunak said millions may be forced into destitution without extra support, as the pair clashed at the penultimate hustings of the Conservative leadership race.

With energy regulator Ofgem expected to raise the price cap to £3,500 a year from October for the average dual-fuel tariff, Truss warned the issue of spiralling fuel costs was not a short-term one. “If people think this problem is going to be over in six months they are not right. This is a long term problem,” she told the audience in Norfolk.

But Sunak said her planned tax cuts would fail to help pensioners and those on lower incomes, and added that extra support for businesses struggling with energy bills was “clearly something the new prime minister will have to look at”.

The two candidates’ economic plans were the focus of Thursday night’s hustings given the threat of a worsening cost of living crisis.

But Truss appeared more at ease, given she is comfortably the frontrunner in polls of Conservative members, and confidently took shots at Sunak, the BBC and the French president, Emmanuel Macron.

The foreign secretary said she would prefer Boris Johnson to be prime minister over her rival in the leadership race. When asked if Macron was a friend or foe, she said: “The jury’s out. If I become PM, I’ll judge him on deeds not words.” And she said anybody who thought the BBC was neutral is “kidding themselves”.

After Sunak claimed the downsides of lockdowns had not been properly considered during the Covid pandemic, Truss went further by arguing schools should never have been shut down and said she raised concerns over whether the government was “too draconian”.

Truss pointed to education when asked by the host, Julia Hartley-Brewer of Talk TV, which public service had improved over the past 12 years of Conservative rule. Sunak struggled to name one, but highlighted the success of the furlough scheme he introduced.

He was more gracious when quizzed on whether Johnson or Truss would make a better prime minister, choosing his rival over the outgoing leader.

Despite going over familiar territory for the two candidates, the issue of the cost of living became more pertinent in light of Ofgem’s impending announcement on Friday.

Sunak said Truss’ national insurance rise reversal and green levy suspension “don’t help” millions of people who are at risk of “falling into destitution”.

He said it was “not credible” for the government to protect everyone, but that he would “go further” by providing extra financial support to pensioners, and those on the lowest incomes – around a third of workers – as well as cutting VAT on energy bills.

“If we don’t do something specific for those people, there’s a high risk that millions of people will fall into destitution,” the former chancellor said. Tax cuts “don’t help” those groups, he added.

Truss admitted there was “a massive issue with people not being able to afford energy”, but added: “What isn’t right is to just bung more money into the system, what we actually need to do is fix the supply of energy.”

She said there should be greater supplies of nuclear and renewable energy, as well as oil from the North Sea.

Truss pledged last night to get people through the energy crisis. Writing in the Daily Mail, she said she would use an emergency budget next month to “ensure support is on its way to get through these tough times”.

“I know how hard it is for millions of Britons, and how grave concerns are about the consequences of today’s decision by Ofgem on the next energy price cap,” she wrote. “The rest of Europe is facing the same challenge, which will loom large as winter sets in.

“If I am elected leader of the Conservative party and prime minister, I will take decisive action on entering No 10 to provide immediate support, but will also tackle the root causes of these issues so we are never again in this difficult position. To those of you feeling the squeeze, my message is clear: I will ensure support is on its way and we get through these tough times.”

The Times reported that Truss this week also held talks with Kwasi Kwarteng, likely to be appointed her chancellor should she win the Tory leadership contest, to discuss emergency support payments for the most vulnerable over winter.

The pair apparently met at Chevening, the foreign secretary’s grace and favour countryside mansion, to consider a multibillion-pound package aimed at pensioners and the poorest households.

Among the measures said to have been discussed were adopting Sunak’s plan to scrap VAT on energy bills and using universal credit to increase help those with larger families or who have disabilities.