Liz Truss’s tax cut-based approach to the energy crisis risks putting millions of Britons in “real destitution”, Rishi Sunak has warned, as the Conservative leadership candidates again clashed bitterly over economic policy.

The hustings event on Thursday in Cheltenham, a key Tory-held marginal seat, also saw the foreign secretary effectively rule out any increased windfall tax on energy firm profits, as she hit out at the “depressing” sight of fields used for solar power.

Sunak’s vehement comments came after Truss, in her own Q&A session at the event, reiterated her belief that tax cuts should be the main response to soaring bills.

Truss told the audience of Tory members this would always be her “first port of call”, followed by a focus on longer-term energy supply issues such as support for fracking and nuclear power.

Also promising she would not call an election before 2024, Truss said she could provide other assistance, but gave no details, saying she “can’t write the chancellor’s budget” before even being elected as prime minister.

“If the answer to every question is raising tax, we will choke off economic growth, and we will send ourselves to penury, and I think that’s a massive problem,” she argued.

Speaking after her, Sunak castigated Truss for what he said was a lack of action to help people who would not benefit from her planned reduction in national insurance contributions, including pensioners and those not in work.

“We are going to, as a Conservative government, leave millions of incredibly vulnerable people at the risk of real destitution,” the former chancellor said. “And I think that’s a real moral failure.”

To do this would mean “the country will never, ever forgive us”, he added.

Truss also said she would “absolutely” not support a wider windfall tax, saying: “I don’t think profit is a dirty word.”

She added: “I think it’s a Labour idea. It’s all about bashing business, and it sends the wrong message to international investors and to the public.”

Asked about an early election, she pledged: “I will not have an election before 2024. I think that’s a very important point, because we’ve got to deliver for people.”

Quizzed about agricultural land, Truss hit out at its use for solar panels: “I think one of the most depressing sights when you’re driving through England is seeing fields that should be full of crops or livestock, full of solar panels.”

She added: “I’m not against solar panels per se. There are plenty of commercial roofs in Britain where we could be putting solar panels. But where they shouldn’t be is on agricultural land that should be used for food production.”

Sunak, widely seen as lagging well behind Truss among Tory members, pitched his appeal as a PM who could appeal to floating voters.

Cheltenham is currently held by Tory MP Alex Chalk, who introduced Sunak, but with a majority of less than 1,000 over the Liberal Democrats.

Asked how he could win over waverers, Sunak explicitly contrasted his style against that of Boris Johnson: “What they want is a government that works competently, seriously, with decency and integrity at the heart of everything it does.

“That’s the kind of government I’m going to lead, and that’s how we’re going to win the election.”