Labour's plans for mass renationalisation will add £150bn to government debt and "risk years of disruption", according to a leading think tank.
The Institute for Fiscal Studies also claimed that returning key parts of the economy to public ownership could make it harder for the party to hit its climate change targets.
Labour has vowed to nationalise the railways, Royal Mail, the water industry, big six energy firms and BT Openreach if it wins the general election.
But in its first assessment of the proposals, the IFS warned that the ambitious programme would be fraught with difficulty and eye-wateringly expensive.
They said it would bring at least 5% of assets currently owned by the private sector into public ownership, increase government assets by more than £200bn and add 310,000 employees to the state workforce.
"It would also bring at least £150 billion of debt onto the public balance sheet on top of the sum paid out to the current owners of these assets in compensation," the IFS said. "The cost of appropriately paying for these companies is uncertain as many of them are privately owned, though it would certainly come, at the very least, to many tens of billions of pounds."
The IFS said the Government must pay the market rate for the industries, warning that failing to do so would damage the UK's international reputation.
And they warned: "Reorganising the ownership and structure of these industries, while simultaneously achieving the ambitious targets that have been set (for instance the rapid decarbonisation of the electricity and gas grids), risks years of disruption."
The think tank also said that the nationalisations may not even be necessary, arguing that Labour's ambitions could be achieved by tightening the regulatory framework that the industries operate under.
IFS research economist Lucy Kraftman said: "At least in the short-run Labour's current plan would lead to significant disruption which could easily, for example, lead to a hiatus in progress towards decarbonisation in the energy sector.
"One should have very clear evidence for the long term benefits before embarking on such a complex and costly set of changes."
But Shadow Chancellor John McDonnell hit out at the IFS, accusing them of pursuing a "nakedly ideological" agenda.
"Labour's proposals for nationalisation will enable us to speed up the transition to a sustainable economy and enable us to meet our decarbonisation targets all the sooner," he said.
"Arguing for sticking to a regulation approach, after years of its failure, is a nakedly ideological stance and one at odds with all the evidence."