An unprecedented operation is under way to refloat the 290-metre liner, which crashed at high speed into Giglio island in a traumatic nighttime accident in January 2012 that left 32 people dead.

But while islanders want the beached ship removed as quickly as possible, the head of Italy's civil protection agency Franco Gabrielli has voiced serious concerns over lingering uncertainties surrounding the ambitious procedure to right it.

Salvage workers hope the vessel will be rolled upright in September, weather and sea conditions permitting – but there are fears that the ghostly wreck could break open during the manoeuvre, spilling a potentially polluting cargo of gallons of cleaning products, rotting food and sewage.

"Even though engineers have performed simulations and come up with hypotheses, we still today do not know how far the rocks have penetrated into the side of the ship, what type of lacerations they caused and what condition the structure is actually in," Gabrielli said.

Without the necessary assurances over the safety of the operation, "the ship will remain in the condition it is in until next year, when good weather conditions will make it possible to ensure it is completely safe," he added.

Italy's environment ministry has launched an inquiry into possible pollution caused by the crash, and on Tuesday divers will take samples of the water underneath the vessel.

The ship's captain, Francesco Schettino, will be in court on Wednesday for the second hearing in a trial against him for multiple manslaughter and abandoning the ship before all of the passengers had been evacuated, after performing a "salute" manoeuvre off the island. He is also charged with causing environmental damage.

Giglio's mayor Sergio Ortelli told AFP the risk of pollution "is non-existent", but the hollowed-out beast instead threatens to damage the island's tourist industry "for a third season" if the salvage operation drags into next summer.

Amid fears of further delays in the removal, he said he was frustrated by receiving verbal promises about the salvage operation's progress and called for ship owner Costa Crociere to commit to a written timetable.

In 2012, visitor numbers to Giglio were down between 28 and 30 percent – though the economic crisis may also be to blame, he said.

The salvage operation, run by Miami-based salvage giant Titan and its Italian partner Micoperi, has had to adapt to difficult conditions encountered while working to stabilize the hulk.