Seven days after the 114,500 tonne liner capsized off the Tuscan coast, its vast wreck is shifting precariously on an undersea ledge, threatening to slide further and undermining plans to pump the oil out safely.

The ship keeled over after striking a rock and is now lying on its side on a shelf in about 20 metres of water off the little island of Giglio. Eleven people were killed and 21 are still unaccounted for.

With hopes of finding any survivors all but gone, experts warn that beyond the loss of lives, this could turn into Italy's worst maritime environmental emergency since the sinking of the Amoco Milford Haven, loaded with 144,000 tonnes of oil, off the coast of Genoa in 1991.

The clean up of that area was completed in 2008, 17 years after the accident, and the Haven shipwreck is still on the seabed, said Luigi Alcaro, head of maritime emergencies at ISPRA, Italy's government agency for the environment.

"If the Costa Concordia slides further down and the fuel begins seeping into the water, we could be talking years and dozens of millions of euros before it can be cleared up," Alcaro told Reuters.

The amount of fuel on board the Costa Concordia, 2,380 tonnes of heavy diesel fuel and lubricating oil, is comparable to that carried by a small oil tanker, Environment Minister Corrado Clini told parliament this week.

The fuel tanks appear to be intact for now.

HIGHLY TOXIC

Clini said even a contained leakage would be highly toxic for the flora and fauna in the area, a natural maritime park noted for its clear waters, varied marine life and coral.

The Giglio island is a renowned diving site and the surrounding archipelago is home to more than 700 botanical and animal species, including turtles, dolphins and seals.

Alcaro said the most optimistic scenario would be to stabilise the ship and pump the oil out through a technique known as "hot tap".

"The oil on the ship is very thick and sticky, so you'd have to drill a hole in the hulk and warm it up to make it more fluid and easier to extract," he told Reuters.

"That could be done in about a month for the 13 external tanks on the ship. There are another 10 tanks inside, and those are a lot more difficult to reach," he said.

But if the ship slips deeper underwater, it would actually be better if the tanks ruptured open and the fuel floated up to the surface, he said.

"There would be panic for a couple of weeks of course but a 'black sea' of fuel would make it visible and easier to recover. The very worst scenario is having oil slowly leaking out."

He pointed to the precedent of the cruise ship Sea Diamond, which sank off the Greek island of Santorini in April 2007, saying oil from the wrecked vessel kept seeping into the water for three years at the rate of 30 kg a day.

Tourism is the top industry on Giglio and locals are worried about the potentially devastating impact of pollution.

"If there's a massive fuel spill, we might as well close everything down, throw away the key and come back in 10 years," said Massimiliano Botti, 40, owner of the Porta Via restaurant along the Giglio quay. "Environmental damage is what concerns us most. If the oil pollutes the coast, we're ruined."

Giglio's mayor Sergio Ortelli said the recovery of the fuel was likely to start within the next 48 hours, but the wreck shifted further on Friday as the weather worsened, forcing a new suspension in the rescue work.

"We can only hope that the weather remains acceptable, that efforts to stabilise the wreck continue speedily, and that God gives us a hand to preserve what many consider a little Mediterranean paradise," Fulco Pratesi, founder of the conservation group WWF in Italy, wrote in the Corriere della Sera newspaper.

(additional reporting by Steve Scherer in Giglio; Editing by Philip Pullella and Janet Lawrence)

• A Permanent Economic Emergency