The Quebec government has given the green light to the construction of a new tunnel between Quebec City and Lévis under the St. Lawrence River, without knowing how much the megaproject will cost when it opens.

The project, known as the “third link” between the two shores, will cost at least $6 billion to $7 billion and its construction will take a decade. Add to that some contingency costs, various “related” charges whose magnitude is difficult to evaluate at the moment, Quebec’s transport ministry says.

For now, the government estimates that additional costs could reach up to 35 per cent of the construction cost of the tunnel, which would bring the total price up to $9 billion to $10 billion.

Details of the project were announced Monday at a news conference with Premier François Legault, Transport Minister François Bonnardel, minister responsible for the region Geneviève Guilbault, Quebec City Mayor Régis Labeaume and Lévis Mayor Gilles Lehouillier.

The tunnel project was a campaign promise of the Coalition Avenir Québec in the 2018 general election. Quebec City has only two fixed links to its south shore — the Quebec Bridge and the Pierre Laporte Bridge, which are right next to each other in the western end of the city. The tunnel, which would become the new easternmost fixed link across the St. Lawrence River, would link the two downtowns.

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