Prime Minister Giorgia Meloni's right-wing government in August approved a 13.5-billion-euro ($15.6-billion) project to build what would be the world's longest suspension bridge across the Strait of Messina.

But in a ruling published late Thursday, the Court of Auditors said the current project violates a European Union directive which protects natural habitats and species.

It said the government had got around the need to ask for the European Commission's opinion on the environmental impact by declaring that the project serves "imperative reasons of overriding public interest".

But it added that these reasons "are not validated by technical bodies and are not supported by adequate documentation".

It also said that under European rules for bidders it would have expected a new tender to be held, considering the sharp rise in costs and change in financing from the original project, which was signed off some 20 years ago.

The World Wildlife Fund (WWF) said it was no surprise to learn "the government's stubbornly pursued procedure violated environmental protection and procurement regulations".

Italian politicians have for decades debated a bridge linking Sicily and the region of Calabria, at the toe of Italy's boot. The bridge would have two railway lines in the middle and three lanes of traffic on either side, with a suspended span of 3.3 kilometres (2.05 miles) -- a world record -- stretching between two 400-metre (1,300 feet) high towers.

The approval in August by a government committee, CIPESS, is the furthest the project has ever got.

Though Rome can push ahead without the Court of Auditors' green light, the government insisted Thursday the issues raised could be clarified.

Advocates say the state-funded project will provide an economic boost for the impoverished south of Italy.

The government also hopes the bridge can be classified as a strategic asset, with its costs counting towards the money Italy has committed to defence spending as part of the NATO alliance.

However, critics have warned over the final cost while there have been local protests over the environmental impact, and complaints that the money could be better spent.