Reform UK has pledged to cut £80bn of public spending within five years by reducing welfare, net zero policies, overseas aid and the number of civil servants.
Speaking from the party's conference in Birmingham, economic spokesman Robert Jenrick said he would like to cut taxes in Reform's first Budget if it wins power at the next election but would have to "earn the trust" of the markets first.
Reform would be the "party of fiscal responsibility", he said and promised to make "big changes... in a careful way, not a cack-handed way".
Labour said the proposed cuts were "incredibly reckless and could mean vital public services like hospitals, schools and law enforcement are put at risk".
The bulk of Reform's proposed cuts come from its previously announced plans to reduce the welfare budget by £50bn.
£10bn would be saved by cutting net zero programmes while £8bn would come from reducing the civil service and £7.1bn from capping the foreign aid budget.
The party says its immigration policies, including abolishing indefinite leave to remain, would result in further savings.
Jenrick is due to give a major speech to the conference on Saturday but speaking at a fringe event with journalist Allister Heath, he sought to reassure voters that Reform's policies would be affordable.
He said it was important to "learn the lessons of [former Prime Minister] Liz Truss" who he said "basically went into a sweetie shop, took everything off the shelves and stuck it into a budget".
"Quite rightly the markets reacted very negatively to that."
Jenrick was a health minister in 2022 when Truss's plan to deliver large energy bill subsidies and £45bn of tax cuts shook market confidence in the UK's financial credibility.
Reform has previously said it wanted to cut inheritance tax but Jenrick suggested that would not be a priority, arguing relatively few people were affected by it.
"It's a bold chancellor that would say I'd rather privilege 36,000 families than 40 million [income] taxpayers," he said.
He said his party would retain the triple lock - the mechanism which ensures pensions rise by either 2.5%, the rate of inflation or the rate of average earnings, whichever is highest.
The triple lock is estimated to cost £15.5bn per year by 2030 but Jenrick said he could not look pensioners "in the eye" and cut their income "while there is so much waste in the public sector".
He said it would be "fundamentally unfair going to pensions before we have made hard choices elsewhere".
Jenrick has written to investors who hold government debt to set up meetings in London and New York to discuss what fiscal rules he would set with the aim of reducing the cost of borrowing.
The past few days have seen global borrowing costs rise, with the UK's costs increasing to a 28-year high.
Also speaking at the conference, the party's business spokesman Richard Tice set out a number of proposals aimed at increasing house building on brownfield sites - land that has previously been built on.
He said the party would consult on measures including scrapping stamp duty, capital gains tax and tax on rental income for a decade for homes built on brownfield sites.
Under the proposals, affordable housing requirements would be removed for brownfield developments outside major cities.
The party's policy document says its plans would reduce tax receipts but argued that additional homes built as a result would boost the economy.
Tice has also suggested cutting business rates for high street shops, offsetting the cost by increasing the tax for online retailers.
Reform's conference at the Birmingham NEC comes at the end of a turbulent summer which saw its leader Nigel Farage resign and then re-run in his Clacton constituency in a bid to end questions about a £5m gift he received before becoming an MP.
There is also a police investigation into donations made to the party.
The main business of the conference is expected to take place on Friday and Saturday, with speeches from Farage and other senior figures, as well as an address by the leader of the French far right National Rally party, Jordan Bardella.
