ROME (Reuters) - A dispute in Italy’s coalition over the future of a high-speed rail link with France escalated suddenly on Thursday and raised the risk of a government collapse, with one party chief accusing his partner of acting irresponsibly.

The Alpine rail line is backed by the ruling League party but is fiercely opposed by its coalition partner, the 5-Star Movement, which argues Italy’s share of the funding would be better spent upgrading existing roads and bridges.

After League leader Matteo Salvini said in an evening television interview he would not back down and his party would “never sign” a decree to block the project, 5-Star chief Luigi Di Maio accused him of threatening to bring down the government.

“He will bear the responsibility before millions of Italians,” Di Maio said in a statement. “I consider this to be irresponsible behavior.”

The TAV project (Treno Alta Velocita) is a joint venture between the Italian and French states to link the cities of Turin and Lyon with a 58-km (36-mile) tunnel through the Alps on which work has already begun.

The European Union has pledged to fund up to 40 percent of costs, Italy up to 35 percent and France up to 25 percent.

Italian Prime Minister Giuseppe Conte said earlier on Thursday that recently updated traffic projections for the line warranted a review of the project’s long-term viability and, if necessary, a renegotiation of the way the funding is split.

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He told reporters he had strong personal doubts about the validity of the venture and he would take responsibility for a final decision based on a cost-benefit analysis already carried out by the government.

That analysis, commissioned by Transport Minister Danilo Toninelli, a 5-Star politician, found the TAV was a waste of public money, estimating the economic return would be a negative balance of 7.0 billion-7.8 billion euros ($7.9 billion -$8.8 billion).

Conte, who is not a member of either ruling party but is closer to 5-Star, called the funding of the TAV “iniquitous” and said he would speak to France and the EU “to share our doubts and perplexities.”

Despite these doubts, he acknowledged the ruling coalition remained “deadlocked” over the issue, as a Monday deadline approaches when the tenders must be launched to build or block key parts of the project.

The right-wing League and the anti-establishment 5-Star have often been at loggerheads since forming an unlikely alliance last year after inconclusive elections.

Conte dismissed media speculation the Alpine rail link could be the issue that finally brings it down as “absurd” and said the dispute between the ruling parties was transparent and constructive.

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