If Liz Truss is elected Prime Minister of the United Kingdom, she will not impose new taxes.
The foreign secretary also hinted that she would consider providing additional assistance to businesses and households facing rising energy costs.
She also ruled out energy rationing, in contrast to her rival Rishi Sunak, who said it should not be considered.
The two were speaking at the final round of the leadership election, which will be held next week.
Economic questions have dominated the race in recent weeks amid rising inflation and soaring energy costs for households and firms.
Both candidates have come under pressure to spell out what they would do to help with energy if they replace Boris Johnson in Downing Street.
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Ms Truss, tipped by pollsters as the favourite in the contest, has said she would reverse April’s National Insurance rise and temporarily ditch energy levies to help people cope with increased costs.
She has not given details of any further help for households but said her chancellor would look to “address the issue of household support” in an emergency Budget she would reportedly hold next month.
Speaking at the hustings in London, the last and twelfth of the contest, she said she would “absolutely be looking to act on business energy costs”.
If she could pledge not to introduce new taxes if she wins power, she replied: “Yes, no new taxes.”
Windfall taxes
She also ruled out any new windfall tax on energy firm profits – something demanded by opposition parties to pay for support to help households with bills.
Asked whether he would do the same, Mr Sunak – who introduced such a tax as chancellor in May – did not say whether he would extend it.
But he said it had been “absolutely the right thing” to introduce a windfall tax when he did, saying firms had made billions in profits due to the effect on energy markets of the war in Ukraine.
Asked if he would rule out energy rationing, he replied: “We shouldn’t rule anything out because the challenges that we face with this crisis are significant.”
Ms Truss also said she would take action to boost the UK’s domestic energy supplies, saying “short-term decisions” had left the UK “dependent on the global energy price”.
She also said she would conduct a “full review of our tax system” – including business rates paid by firms and the tax burden on families.
Elsewhere at the London hustings:
Ms Truss said the “experiment” in all-lane running smart motorways – which use the hard shoulder as a live traffic lane – “hasn’t worked”, and she would get rid of them where they are not working
She also said she was “prepared to look at” introducing advisory speed limits on motorways
Mr Sunak defended his plan to introduce £10 fines for missed GP appointments, saying it was required to “change the culture in this country, so that’s not acceptable”
He also suggested Britons should overcome their “aversion to flat-pack housing” as a potential solution to deliver more homes
Mr Johnson’s successor will be announced on Monday, and he or she will take over as the new occupant of Downing Street the next day.
In a break with tradition, the winner will travel to Balmoral Castle in Scotland to be appointed by the Queen. She has been suffering from mobility issues, and it is understood the change was made to prevent the need for any last-minute rearrangements.