Billions have been announced to replace lost police officers, end the benefits freeze, boost defence, extend childcare and rebuild technical education, in recent weeks.
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“If they are planning further increases in day-to-day spending, that would not be possible without tax increases or other cuts in spending,” said Isabel Stockton, its research economist.
The conclusion comes amid criticism of the chancellor for ducking scrutiny of his spending plans by pulling out of a TV debate, while an update on the economy by the Treasury watchdog was axed.
Meanwhile, the Conservative election manifesto will not be published until as little as two weeks before polling day on 12 December – long after millions of people have cast postal votes.
However, the IFS also has strong criticism of Labour, which has claimed its much more lavish spending plans can be funded purely from tax hikes on big earners and big companies.
Ms Stockton added: “It’s difficult to raise enough money for substantial increases in spending solely from high-earners – you need to look to a broad-based tax.”
The chancellor is boxed in because, despite loosening the purse strings for investment spending, he has promised to balance the budget for day-to-day spending within three years.
In a much-criticised interview – in which he was mocked by a live audience – the Treasury chief secretary claimed the Tories could achieve a “£30bn surplus over the next few years”.
But the IFS says this has “almost all” disappeared, partly because of the £13.8bn to be spent in the next financial year, and also because of £18bn of classification changes, including on student loans and public-sector pensions.
On the alleged £37bn of “headroom”, Ms Stockton said: “It’s important to consider that the forecast made in March is no longer a very good guide to what we now know.
“The changes are enough to wipe out almost all of what looked like a healthy surplus back in March.”
She added: “If they want to have more police officers, for example, or have other aspirations, they would have to be paid for.”
However, far from planning tax increases, Boris Johnson has the headache of trying to deliver huge tax cuts for top-earners that he promised to win the Conservative leadership.
Mr Johnson said he wanted the threshold for paying the 40p income tax rate hiked from £50,000 to £80,000, at an eye-watering cost of £9bn.
He repeated his intention to lower taxes in a speech on Wednesday, telling his audience: “Let’s go for sensible, moderate, One Nation, but tax-cutting, Conservative government, and take this country forward.”
Mr Javid, meanwhile, told the Conservative conference in September of his wish to scrap inheritance tax, at a cost of £5bn, saying: “It’s something that’s on my mind.”
Spending increases for health and education were announced before the election campaign, and can be afforded, the IFS says. But other expensive – and uncosted – pledges have followed.
Ending the controversial four-year benefits freeze, blamed for an increase in poverty, will, along with state pension hikes, cost around £5bn a year.
Only 6,000 of Mr Johnson’s promised 20,000 extra police officers will be funded next year, with several billion still to be found for the majority of recruits.
Defence spending is due to rise by £2.2bn over two years – which means one year’s funding is still to be found – and a “revolution” in technical education has also been promised.
Mr Johnson himself pledged that the 30 hours of free childcare for three- and four-year-olds would be available for 48 weeks a year, up from 38 weeks, at a cost of around £600m. The Conservatives are also considering a more expensive pledge, costing £700m, to offer free childcare for all two-year-olds.
The analysis that the chancellor has money for his previous pledges excludes the risk of a downturn after Brexit, amid warnings that Mr Johnson’s proposals for future trade will cost the country many billions.
The Treasury declined to comment on the IFS analysis, but referred The Independent to Mr Javid’s previous insistence that he still had significant headroom to increase spending without hiking taxes.
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