"The TAV must absolutely be built, especially as it would cost more to scrap it than to build it," the head of the League said during a Rai2 programme due to air on Friday.
"I'm still waiting for the damn cost-benefit analysis, of which I have yet to see one page," he added, according to quotes from the pre-recorded programme published in the Repubblica daily.
The high-speed line between Turin and Lyon has become a key battleground for the populist government in Rome, with Salvini's coalition partner, the Five Star Movement (M5S), demanding the project be scrapped.
The tunnel is planned to run for more than 50 kilometres underneath the Alps. Photo: Marco Bertorello/AFP
The €8.6-billion tunnel through the Alps, which has already been partially dug, will reduce travel time between Milan and Paris from almost seven hours to just over four.
Proponents of the line, launched nearly 20 years ago and officially scheduled to be finished in 2025, argue that it will rid the roads of a million trucks and avert some three million tonnes of greenhouse gas emissions each year.
But as well as being attacked by environmentalists, the line has been criticised as a misuse of public funds and the anti-establishment M5S had pledged to block it if elected.
Although the League has long favoured the venture, Salvini had said 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 been forced to break since coming to power, with analysts warning of the potential fall-out particularly for populist leader and deputy prime minister Luigi Di Maio.
Photo: Marco Bertorello/AFP