Tory leadership contenders have made spending commitments worth at least £330bn after just two days of the campaign, according to analysis by Labour.
Eleven candidates have so far entered the race to succeed Boris Johnson as prime minister.
Former chancellor Rishi Sunak, who is the frontrunner, has warned Tory members not to believe “fairy tales” about unaffordable tax cuts.
But his rivals appear to be locked in an arms race over who can pledge to introduce the biggest tax cuts.
Launching her campaign, foreign secretary Liz Truss pledged to cut taxes “from day one” by scrapping the rise in national insurance brought in by Sunak.
Nadhim Zahawi, who replaced Sunak as chancellor last week, today said he would cut the basic rate of income tax from 20p to 19p in the pound next year, and to 18p in 2024, while also pledging to reduce the overall tax burden as a percentage of GDP every year.
Sajid Javid, another ex-chancellor, has he would cut a penny off income tax next year, reduce corporation tax to 15 per cent and also scrap the rise in national insurance payments.
Transport secretary Grant Shapps, meanwhile, has vowed to bring in an emergency budget to immediately cut 1p off the basic rate of income tax if he becomes prime minister, reverse the national rise and also increase defence spending to 3 per cent of GDP.
A dossier produced by Labour said the pledges made by all of the Tory hopefuls would cost at least £330bn.
Speaking in Gateshead, Keir Starmer said: “That’s more than the annual budget of the NHS, splurged on to the pages of the Sunday papers, without a word on how it’ll be paid for.
He challenged candidates to “tell us precisely where they’re getting that money from”, whether it is from cuts to public services or increases to borrowing.
“Or is it the magic money tree that they’ve suddenly discovered having arrived back down here from the moon where they’ve been for the last 12 years?” he said.
Lord Lamont, the 1980s Tory chancellor, also warned against “unfunded, irresponsible tax cuts”, which he said would drive up interest rates.
He told Radio 4’s World at One programme: “I’m increasingly concerned because I think there is a danger that this leadership election is going to descend into a sort of Dutch auction of tax cuts which are not necessarily affordable, not necessarily rightly timed.
“There is a danger at this point when the public finances, the amount we are borrowing, is not in a strong state.”
The Conservative peer added: “You can’t grow your way out of inflation, you’re just likely to add to it if you attempt to do that.
“I don’t suppose many people want to see interest rates above the level of inflation but if we start giving unfunded, irresponsible tax cuts the Bank of England will be faced with difficult choices indeed.”