An unprecedented bid to refloat the giant Costa Concordia cruise ship from its watery grave off Italy has begun, with a delicate operation to raise the crippled vessel two-and-a-half years after it crashed.

“I am a bit nervous,” said Salvage Master Nick Sloane on Monday as he headed out to the wreck off the Tuscan island of Giglio, where he will oversee the first-ever operation to float a ship of this size from an onboard control room.

“It’s a very complex operation,” Franco Gabrielli, head of the civil protection agency overseeing the salvage, told a crowd of reporters on the island on the eve of the start.

Spectators gather to watch the Costa Concordia be refloated. Photo: AAP

The port is a hive of activity, with curious holidaymakers and local residents watching a 350-strong team of divers and engineers carrying out final checks of the ship.

The liner will be refloated over a week and then be towed away to Genoa port in northern Italy for scrapping.

The most delicate part of the venture will be on Monday, when the 114,500-tonne vessel is raised two metres off the artificial platform it has rested on since it was righted in a dramatic operation in September.

Air will slowly be pumped into 30 tanks or “sponsons” attached to both sides of the 290-metre Concordia to expel the water inside and raise the ship.

It will then be towed away from the shore and moored using anchors and cables.

“The risks are that the ship could bend as it is raised, or the chains underneath it could snap,” Nick Sloane, a globe-trotting South African salvage master who is in charge on the technical side of the operation, told AFP.

“There will be 42 people on board during the first manoeuvre. If disaster strikes we will evacuate through emergency escapes on the bow and stern,” said Sloane, who will oversee from a control room on the Concordia itself.

The whole operation is expected to take around six hours.

If all goes well, all the sponsons will be lowered into position on Tuesday and Wednesday.

“That will be the point of no-return,” senior engineer Franco Porcellacchia said.

“It’s an unprecedented operation and, as with anything being done for the first time, there are risks. But we are confident,” Porcellacchia said.

The Concordia – twice as big as the Titanic – crashed off Giglio on the night of January 13, 2012, forcing many of its 4,229 passengers and crew from 70 countries to jump into the sea.

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