Liz Truss's tax-cutting plans have been condemned as an "electoral suicide note" for the Conservative Party by a senior backer of her rival, Rishi Sunak.
UK Deputy Prime Minister Dominic Raab took aim at the foreign secretary's emergency budget plans in one of the most strongly worded interventions of the leadership race so far.
With Ms Truss on the back foot over the cost-of-living crisis, Mr Sunak promised, if elected prime minister, to unveil more support for households once the size of autumn energy bills becomes clear.
The two candidates will set out their pitches at the latest round of hustings on Tuesday in Darlington, a constituency in the north of England which switched from Labour to the Conservatives at the last election.
A Truss backer, Defence Secretary Ben Wallace, criticised the Tory infighting on a campaign stop in Yorkshire by saying that "blue-on-blue is not going to help the Conservative Party win the next election".
Ms Truss used the visit to an aerospace company in Huddersfield to promise a tough line on defence, repeating a promise to increase military spending to 3 per cent of GDP.
But polls show the economy is voters' top concern two years from the next general election, as fuel prices drive the highest inflation in decades and economists predict a recession to add to Britain's woes.
Analysts Cornwall Insight said on Tuesday that the price cap is expected to soar to around £3,582 ($,4,340) per year in October, from £1,971 ($2,388) today, before rising even further to about £4,200 ($5,088) in the New Year.
Ms Truss, who leads Mr Sunak in polls of the Tory race, came under pressure after telling the Financial Times that she would tackle the problem "in a Conservative way of lowering the tax burden, not giving out handouts".
She and her allies have since sought to clarify that further government support was not being ruled out. Ms Truss said on Tuesday that she does not believe in “taxing people to the highest level in 70 years” and then “giving them their own money back”, in a thinly-veiled swipe at Mr Sunak's record as chancellor.
But Mr Raab wrote in The Times that focusing on tax cuts "isn't Conservative politics … it's bad politics".
"If we go to the country in September with an emergency budget that fails to measure up to the task in hand, voters will not forgive us as they see their living standards eroded," he said.
"Such a failure will read unmistakably to the public like an electoral suicide note and, as sure as night follows day, see our great party cast into the impotent oblivion of opposition."
Mr Raab's turn of phrase is most commonly associated with the 1983 Labour Party manifesto, a hard-left document described at the time as the "longest suicide note in history".
Penny Mordaunt, a former leadership candidate who now backs Ms Truss, defended the foreign secretary by saying Ms Truss "wants to do more" and that extra support was not being ruled out.
Conservatives on the campaign trail - in pictures
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Candidates Rishi Sunak and Liz Truss at the Conservative Party leadership election hustings at Wembley Arena, London. This was the final hustings attended by Tory Party members who will vote for the new leader and next prime minister. EPA -
Liz Truss addresses Conservative Party members at Wembley Arena. AP -
Mr Sunak puts forward his case to be leader while speaking at Wembley Arena. Bloomberg -
Mr Sunak visits his family's old business, Bassett Pharmacy in Southampton, while on the campaign trail. Reuters -
Ms Truss speaks to staff at Condimentum Ltd at the Food Enterprise Park in Norwich. Getty Images -
A Rishi Sunak supporter at a hustings event in Norfolk. PA -
Mr Sunak greets people at a leadership hustings in Manchester. Bloomberg -
Ms Truss poses for a selfie with a supporter as she arrives for the event in Manchester. Getty -
Rishi Sunak looks through the sights of an anti-tank missile launcher, supplied to Ukraine, during a visit to the Thales Defence System plant in Belfast, Northern Ireland. AP -
Liz Truss at a Conservative Party leadership campaign event in Belfast. Getty -
Liz Truss meets supporters before a hustings in Perth, Scotland. Getty -
Rishi Sunak speaks during a campaign visit to Cluny Castle in Inverurie, Scotland. Getty -
Police hold back protesters outside Perth Concert Hall, Scotland, where Conservative leadership candidates Liz Truss and Rishi Sunak were speaking. PA -
A mural showing the two leadership contestants appears on a wall in Belfast, Northern Ireland. AP -
Rishi Sunak during a visit to St John's Wood Synagogue, north London. PA -
Liz Truss at the Conservative Party leadership election hustings in Cheltenham. EPA -
Rishi Sunak during a hustings event in Cheltenham. Reuters -
Liz Truss speaks to scientists during a campaign visit to a life sciences laboratory at Alderley Park in Manchester. PA -
Rishi Sunak during a campaign hustings in Darlington, England. Bloomberg -
Liz Truss meets party members before she speaks at the Darlington campaign hustings. Getty -
Rishi Sunak arrives for the Darlington hustings. Getty -
Liz Truss plays pool during a visit to the Onside Future Youth Zone in London. Reuters -
Rishi Sunak looks at a book with Teddy Openshaw, 4, as his father, Henry, looks on after a Conservative leadership campaign event in Ribble Valley, Lancashire, England. Getty -
Liz Truss plays table tennis with former Conservative party leader Iain Duncan Smith during a visit to the Onside Future Youth Zone in London. Reuters -
Liz Truss with supporters at a campaign event in Solihull, England. Reuters -
Rishi Sunak attends a campaign event held by his party in Bexhill, England. Reuters -
A protester is removed as Ms Truss speaks during the Conservative Party leadership hustings in Eastbourne. Bloomberg -
Mr Sunak speaks at an event in Edinburgh. PA -
Ms Truss leaves the hall in Exeter, south-west England, after her speech to a Conservative Party membership hustings. Getty -
Mr Sunak speaks at the University of Exeter in south-west England. AP -
A supporter of Ms Truss at a Conservative Party hustings in Exeter, south-west England. Getty -
Ms Truss at a hustings event at the Pavilion conference centre at Elland Road in Leeds. PA -
Mr Sunak in Salisbury watching a screening of the Uefa Women's Euro 2022 final between England and Germany at Wembley stadium. England won 2-1 in extra time. PA -
Ms Truss before the Uefa Women's Euro 2022 final at Wembley stadium in London. AFP -
Mr Sunak meets Tory members at Fontwell Park Racecourse. Reuters -
Ms Truss at an event in Dereham, Norfolk. AFP -
Mr Sunak in Ropley, near Winchester, Hampshire. PA -
Ms Truss speaks as Conservative Member of Parliament Tom Tugendhat looks on at a Conservative Party leadership campaign event at Biggin Hill Airport. Getty -
Ms Truss on the campaign trail in Woodford Green, on the outskirts of London. Reuters -
Mr Sunak at a Conservative Party hustings event in Leeds. AFP -
Ms Truss speaks at the hustings event in Leeds. AFP -
Sunak and Truss-themed souvenirs at a stand during the Leeds hustings event. Reuters -
Ms Truss, and her mother, Patricia, arrive for the hustings event. Getty -
Ms Truss poses with supporters in Morley, England. Getty -
Mr Sunak delivers a speech during a campaign event in Newmarket. AFP -
Ms Truss and Jake Berry, MP for Rossendale and Darwen, during a visit to a broadband interchange company in Leeds. PA -
Mr Sunak answers questions as he takes part in the hustings in Leeds. AFP -
Ms Truss speaks at an event at a private house with members of the Conservative Party in Woodford Green. PA -
Ms Truss and Mr Sunak pose for a picture before a BBC leadership debate at Victoria Hall in Hanley. Getty -
Ms Truss addresses Mr Sunak during the televised debate. Getty -
Mr Sunak, with daughters Krishna and Anushka, and wife Akshata Murthy, during a visit to Vaculug tyre specialists at Gonerby Hill Foot, Grantham. PA -
Ms Truss speaks to people while campaigning in Marden. Getty -
Ms Truss celebrates after being named, along with Mr Sunak, one of the final two candidates in the race to become the UK's next prime minister. PA -
Mr Sunak outside his campaign office in central London. EPA -
Ms Truss outside Downing Street in London. Reuters -
Rishi Sunak meets Conservative Party members and activists, in Teesport, Redcar. Reuters -
Conservative leadership candidates Mr Sunak, Ms Truss, Kemi Badenoch, presenter Julie Etchingham, Tom Tugendhat and Penny Mordaunt during 'Britain's Next Prime Minister: The ITV Debate' in London. EPA -
Ms Badenoch arrives at Here East studios in Stratford, east London, before a live television debate. PA -
Mr Sunak arrives at a London event to launch his campaign to be the next Conservative Party leader and UK prime minister. Reuters -
Mr Tugendhat appears on the BBC One current affairs programme, 'Sunday Morning'. PA -
Ms Badenoch, Ms Mordaunt, Mr Sunak, Ms Truss and Mr Tugendhat before the live television debate. PA -
Ms Mordaunt arrives for the launch of her Conservative leadership campaign at the Cinnamon Club in Westminster. PA -
Ms Truss speaks at the launch event for her campaign to become the next leader of the Tory Party and Britain's prime minister, in London. EPA -
Chancellor of the Exchequer Nadhim Zahawi, one of the candidates who has been knocked out of the race, giving a speech at the Churchill War Rooms. PA -
Former leadership candidate Jeremy Hunt appearing on the BBC One current affairs programme, 'Sunday Morning'. PA -
Sajid Javid attending the launch of his campaign to be Conservative Party leader and prime minister, at the Cinnamon Club in London. He has since been knocked out of the contest. PA -
Rehman Chishti is interviewed after announcing his bid to become the next prime minister. Mr Chishti is no longer in the race. Getty -
Mr Tugendhat speaking at the launch of his campaign. PA -
Culture Secretary Nadine Dorries and Conservative MP Iain Duncan Smith arrive for the Truss campaign launch event. Getty -
Former leadership candidate Attorney General Suella Braverman at the Churchill War Rooms in central London. PA -
Supporters of Mr Sunak wait for his arrival ahead of the campaign launch. Bloomberg -
Mr Tugendhat makes a speech at the launch of his campaign. PA -
Mr Javid mops his brow while launching his campaign. Getty -
Ms Truss leaves for work after her announcement that she would be running for the leadership of the Conservative Party. Getty
But Mark Harper, another Sunak backer on the Conservative benches, told Sky News that cutting taxes would not do much for poorer people who pay little money to the Treasury anyway.
"There’s a very big difference between large, permanent, unfunded tax cuts that go to the wealthiest in society, rather than targeted help," he said.
Mr Sunak said he would aim to keep any one-off borrowing to an “absolute minimum” by seeking efficiency savings across government departments.
He said he would announce details of new support once energy regulator Ofgem reveals the autumn price cap at the end of this month.
Ofgem said last week that the price cap would now be raised every three months, rather than every six, because it is "not in anyone’s interests for more suppliers to fail and exit the market".
