The final two politicians vying to be the next British prime minister have come to blows over their fiscal proposals, with Rishi Sunak warning against a "huge borrowing spree" as Liz Truss defended tax-cutting plans worth at least £30 billion a year.

Mr Sunak, the former chancellor, did not pull any punches in an interview yesterday evening, claiming the current evidence suggests the Conservatives would suffer a defeat at the next general election under his rival's leadership.

Mr Sunak said he thought borrowing £30 billion for unfunded tax cuts would be "inflationary", adding that going on a "huge borrowing spree" would only "make the situation worse".

Speaking on LBC's Tonight With Andrew Marr, that "if you look at all the polling evidence that we have, and you see what that says, it's pretty clear that I am the person that is best placed to defeat Keir Starmer in the next election".

Asked if that meant the Conservative Party would likely be defeated in the next election if Ms Truss became leader, he replied: "That's what all the evidence that we have today would show, and that's what our members will need to consider".

Earlier, Ms Truss defended her tax cut plans as "affordable", as the economic policies of the two candidates came under scrutiny.

"What is not affordable is putting up taxes, choking off growth, and ending up in a much worse position," she told broadcasters during a visit to Peterborough.

Mr Sunak was understood not to be envisaging cutting personal taxes until at least autumn next year to avoid fuelling inflation.

Rishi Sunak is campaigning to become the UK's next prime minister

However, Ms Truss, the Foreign Secretary, promised an emergency budget to reverse the national insurance hike immediately under her proposals to drive growth.

The financial plans of the final candidates for prime minister were growing more divided as they battled for the votes of the Tory membership required to win the race for No 10.

Robert Joyce, the deputy director of the Institute for Fiscal Studies (IFS) think tank, put Ms Truss's tax cuts at "more than £30 billion per year - and possibly considerably more".

The plans "mean higher borrowing or less public spending, or some combination", he said, though their impact remains unclear because Ms Truss's proposals are "yet to be fleshed out".

Liz Truss has pledged tax cuts

Ms Truss insisted she was not planning "public spending reductions" despite pledging vast tax cuts.

"What I am planning is public service reforms to get more money to the front line, to cut out a lot of the bureaucracy that people face," she said.

"I'm certainly not talking about public spending cuts. What I'm talking about is raising growth."

Mr Sunak told LBC he wants to "restore trust in government" as prime minister.

He said his life would be easier if he vowed to do "this lovely sounding thing and this lovely sounding thing", but argued that would not be leadership.

Mr Sunak was the parliamentary party's favourite, winning 137 votes to Ms Truss's 113 among Tory MPs.

But bookmakers placed the Foreign Secretary as the frontrunner, with early indications suggesting she is more popular with Tory members ahead of a summer of campaigning.

The pair yesterday battled to win the support of local politicians when they took part in private hustings for the Conservative Councillors' Association.

They will tour the UK to take part in 12 hustings for the Tory members who will vote for their next leader, with the result being announced in September.