The refloating of Italy's ill-fated Costa Concordia cruise ship is set to begin Monday, with a high-risk salvage operation to raise the rusting hulk ahead of its final journey to the shipyard where it was built.

Two and a half years after it sank off the island of Giglio in a nighttime disaster that left 32 people dead, the plan is to raise and tow away the 114,500-tonne vessel in an unprecedented and delicate operation.

"The most critical phase will be the first day, raising the wreck for the first time. Refloating a passenger ship this large has never been attempted before," South African Nick Sloane, who is in charge of the salvage, told AFP.

The 290-metre (950-foot) ship was rotated upright in September and is now sitting on an underwater platform.

Thirty tanks or "sponsons" that have been welded to the sides will work as a pneumatic system to raise it.

Under the gaze of the world's media, the operation is expected to start on Monday with workers pumping compressed air into the sponsons to lift the Concordia by two metres.

The environmental committee overseeing the operation gave a preliminary go-ahead on Saturday but said that final confirmation would come on Sunday, depending on the weather.

The operation is to begin at 6:00 am (0400 GMT) on Monday.

In a worst-case scenario, some environmentalists warn the hull could break apart and spill its rotting innards into what is one of Europe's largest marine sanctuaries.

"We're talking about a floating city kitted out for thousands of passengers, with gallons of pollutants such as oils, detergents and sewage chemicals still inside," said Giorgia Monti from Greenpeace, which is sending an observation team to monitor the operation.

After the initial lift, tug boats would drag the wreck 30 metres (32 yards) east and secure it in place before it is slowly buoyed another 10 metres, with engineers checking each deck for fresh structural damage as they emerge.

All maritime traffic in the area -- which includes a popular beach resort -- will be blocked during the most delicate phases.

A team of experts will manage the operation from a control room under the guidance of Sloane, who has described the salvage as his "most challenging" yet in a career that has taken him to six continents and two warzones.

- Journey fraught with hazards -

Ship owner Costa Crociere said on Saturday that the whole procedure is expected to take six or seven days.

"The departure of the Concordia from Giglio is currently scheduled for July 21," the company said in a statement.

"It is a complex operation never attempted before, but we know we can count on the best technicians in the world," Costa Crociere's chief executive Michael Thamm was quoted as saying.

Salvage operators were expected to deliver a technical briefing on Giglio Island on Sunday but they have already said the Concordia would be towed at two knots an hour some 240 kilometres (149 miles) north to the port of Genoa.

It is expected to arrive later this month to be scrapped near the same shipyard it was built and launched in 2006.

Salvage costs so far are estimated at around 1.1 billion euros ($1.5 billion), including 100 million euros for the scrapping.

The luxury liner -- twice the size of the Titanic -- crashed into rocks just off tiny Giglio island in January 2012 and keeled over with 4,229 people from 70 countries on board.

The ship's captain Francesco Schettino is currently on trial for manslaughter, causing a shipwreck, and abandoning the vessel before all of the passengers had been evacuated.

Four other crew members and a Costa Crociere executive have plea-bargained and the company has accepted limited responsibility as Schettino's employer.

The body of one of the victims, Indian waiter Russel Rebello, is still missing and his remains may be found during the refloating or dismantling of the vessel.

The four-day journey to Genoa is fraught with possible environmental hazards, with warnings that some of the 100 tonnes of fuel and 263,000 cubic metres (69.5 million gallons) of polluted water flooding its lower decks could leak out.

Costa Crociere insists the amount of leakage will be comparable to that discharged by any vessel crossing the area -- one of the most trafficked in the Mediterranean.