Italy's government is divided over the proposed TAV (Treno Alta Velocità) line between Turin and Lyon.
The plan is supported by Interior Minister Matteo Salvini's League but rejected by its coalition partner. the Five Star Movement (M5S).
The report, made public today, was aimed at helping the government settle the matter.
It said the trans-Alpine train line and tunnel would be "very negatively profitable", losing seven billion euros by 2059.
While the line could make 1.3 billion euros from passenger rail traffic, that does little to offset future construction and management costs of 7.9 billion euros, plus 463 million euros in lost revenue from motorway tolls and lost fuel taxes, the report said.
The tunnel through the Alps, which has already been partially dug, would reduce travel time between Milan and Paris from almost seven hours to just over four.
Drilling the tunnel beneath the Alps began more than a decade ago. Photo: Jean-Pierre Clatot/AFP
Proponents of the line, launched nearly 20 years ago and officially scheduled to be finished in 2025, argue that it would rid the roads of a million trucks and avert some three million tonnes of greenhouse gas emissions each year.
But the link has been criticised as a misuse of public funds. and during election campaigning the Five Star Movement (M5S) had pledged to block it if they came to power.
Although the League has long been in favour of the venture, Salvini had previously said the project "must go ahead", later saying he would wait to see the cost-benefit analysis before adopting a position.
The M5S fears the so-called TAV may join the growing list of promises it has broken since coming to power, with analysts warning of the potential fall-out particularly for populist leader and deputy prime minister Luigi Di Maio.
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The line and its 57.5 kilometre tunnel have become yet another bone of contention between Rome and Paris, which last week recalled its ambassador in an escalating war of words between the two governments.
Italy's Transport and Infrastructure Minister Danilo Toninelli described the report's figures as "merciless" but also as the beginning of a dialogue with Paris.
Italy's centre-left opposition Democratic Party noted that five of the six experts who drew up the report had previously slammed the project. The sixth expert refused to sign off on the report.
A committee in charge of the railway line, based in France, said the analysis was "remarkably partisan" as it described the project's environmental benefits such as less road travel and pollution as economic losses through less petrol tax and motorway toll revenue.
The European Union, which has already pumped hundreds of millions of euros into the project, said recently that it could ask for its money to be repaid if work is halted.