Unfunded spending promises announced by both parties mean they are likely to have to raise taxes further or cut spending if they are to meet their targets, the Resolution Foundation said.
The think-tank warned that the approach risked damaging the UK's economic credibility, saying the parties had breached their own spending rules within days of announcing them.
In a bid to convince voters that they can be trusted with the public finances, Sajid Javid, the chancellor, and John McDonnell, the shadow chancellor both set out strict spending rules on 7 November - days before the two parties announced their respective spending plans.
The Tories have promised to balance the UK’s current budget – the government’s day-to-day spending – within three years, and cap borrowing for investment at 3 per cent of GDP.
The Resolution Foundation study found that the party is unlikely to be able to stick to these rules because it has not said how it would fund a £6bn commitment to raise the employees' National Insurance threshold.
The Tories' tiny amount of spending headroom means that, if there was even a tiny drop in the national finances, they would likely have to raise taxes or slash spending again if they are to meet their targets, the report said.
Labour, meanwhile, has vowed to balance the current budget within five years. The Resolution Foundation said it would struggle to meet this target because it had failed to account for £10bn of interest and depreciation costs from is spending plans.
The party’s £58bn commitment to compensate the “Waspi” women - those born in the 1950s who lost out because of changes to the state pension age - would blow its fiscal rules out of the water, the report said.
To meet its rules, a Labour government would either need to ditch several major spending commitments or else raise taxes even further, it added.
The Resolution Foundation had previously urged the parties to stick to rules similar to those that they have now adopted, but said both the Tories and Labour “seem all too willing to bend or even break their own fiscal rules before they’ve even got started”.
James Smith, the think-tank’s research director, said: “During this election, all the main parties have set out new fiscal rules that would frame their stewardship of the economy. They are right to have adopted new approaches that avoid some of the pitfalls of previous narrow focuses on debt levels.
“However, the parties seem all too willing to bend or even break their own fiscal rules before they’ve even got started. Both parties have failed to account for the wider costs of their very significant investment programmes."
He added: “The Conservatives have left themselves almost no fiscal headroom, and would fail to balance the books with even the smallest deterioration in the economic outlook. Labour meanwhile already look to have broken one of their rules with a £58 billion unfunded spending commitment to WASPI women.
“After a decade of austerity, the desire of politicians to move on and invest in our economy is understandable. But taking huge risks with the brand new fiscal rules that are supposed to bind the next government for its whole time in office risks seriously undermining the UK’s economic credibility, at a time when it is already under strain.”
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