The independent Institute for Fiscal Studies said there were “genuinely big differences” between the rival approaches on the key battleground issue but urged clarity on the tax and spending measures.
It delivered ammunition for each side — suggesting the Tories would have to make swingeing cuts to services and Labour would borrow £26bn (€36bn) a year.
And in a blow to the Scottish National Party, it said there was a “significant disconnect” between its claims to be an anti-austerity party and plans that implied it would be spending less than Labour by 2019/20.
“They would cut less to start with but the implication of the plans they have spelt out in their manifesto is that the period of austerity would be longer than under the other three parties we consider,” it said.
SNP leader Nicola Sturgeon has accused Ed Miliband of offering a “Tory-lite” economic plan.
The thinktank’s verdict was delivered amid renewed hostilities. Labour said Tory cuts would be bigger than in any of the world’s advanced economies.