Tory leadership hopeful Rishi Sunak has pledged to scrap VAT on household energy bills if prices continue to rise - a surprise move which could cost more than £4billion.
The former chancellor, who had not promised tax cuts so far in his campaign, said the plan would help families with rising living costs.
The BBC says rival Liz Truss's campaign team described the step on VAT as desperate - accusing him of a "screeching U-turn".
Meanwhile Ms Truss has pledged to cut key crime rates if she wins.
The latest pledges come after a live TV debate between the two contenders on Tuesday night was halted and taken off air halfway through when presenter Kate McCann fainted.
The VAT announcement marks a tactical shift from Mr Sunak, who is trailing in opinion polls of Conservative party members, who will select the winner when they vote over the next month..
Under his "winter plan", the 5% VAT rate on household energy would be scrapped for one year from October, if the price cap on bills rises above £3,000 for the typical household.
Tax cut
He added that the "temporary and targeted" tax cut would save average households £160 a year, and ensure people get "the support they need" while also "bearing down on price pressures".
It contrasts with his approach as chancellor, when he argued against a Labour plan to scrap energy VAT earlier this year and said the policy would "disproportionately benefit wealthier households".
Mr Sunak, who has accused Ms Truss of making unfunded tax cuts that amount to £55billion, said his plan would cost £4.3billion but argued that as it was temporary it could be delivered within existing borrowing rules.
He has also promised to double the number of hours benefit claimants have to work before they can stop attending appointments with work coaches from nine to 18 hours per week.
Work and Pensions Secretary Therese Coffey, a Truss supporter, argued Mr Sunak had blocked a move to raise the limit to 12 hours earlier this year.
In announcements of her own, Foreign Secretary Mr Truss said she would set police forces a target of cutting homicide, serious violence and neighbourhood crime by 20% before the end of the current Parliament.