The giant steel and concrete box which is seen as the best chance of stopping the BP oil spill arrived at the disaster scene in the Gulf of Mexico today, as authorities struggled to control oil breaching an island wildlife preserve.

Over the next few days cranes will attempt to lower the 100-tonne contraption around 5,000ft (1,500 metres) to the sea floor and position it over a leaking pipe that has been gushing 210,000 gallons of crude a day into the Gulf.

If all goes accord to plan, the four-storey box will serve as a giant funnel, collecting the oil and piping it to a ship.

BP officials have said repeatedly that the plan for capturing and redirecting the oil is exceedingly complex, and has never before been tried at such depths. "You have 100 tonnes of steel. You have to stop, reposition, and stabilise these sorts of things. I don't think it's a fast process," said John Curry, a BP spokesman.

He said the system had been successfully deployed at depths of 365 metres after hurricane Katrina, but not by a BP crew. In a best-case scenario, the contraption should be operational by Monday. One bookmaker rated the chances of shutting off the leak completely at 1,000-1.

The US coastguard confirmed for the first time that oil had made its way past protective booms and was surrounding Freemason Island. The area is part of a chain of uninhabited barrier islands in the Breton national wildlife refuge.

Meanwhile, the Obama Administration suspended planned lease sales for drilling off the coast of Virginia.

Environmentalists who have visited the area described a thin band of orange-coloured oil lapping at the shore. "The expanse itself is made up of three to four different kinds of oil ranging in thickness, including bright orange ribbons of weathered oil on the surface," said Rick Steiner, a conservation professor at the University of Alaska, who flew over the region for Greenpeace.

"There were no visible research vessels, and the only visible relief effort in this area was about 30 shrimp boats off the shore of Little Gozier island pulling booms, with no skimmers taking the oil off the surface."

Efforts to hold back the oil by repositioning booms were undermined by BP's decision to break up the spill with dispersants. The resulting thinner oil was proving much harder to contain.

"The oil just scoots right under the boom. There is as much oil behind the booms as in the booms," he said. "It's an exercise in futility."

The breach deepened fears for brown pelicans and other endangered birds. "This is an important nesting season," said David Ringer, of the Audubon Society.

Heavier concentrations of crude remained further offshore, and the coastguard said weather forecasts suggested it would remain so until the weekend.

BP said it was relying on cameras from submersible robots to help guide the containment box into place and avoid further damage to the pipe.

In addition to the dark, frigid temperatures and extreme high pressures at such depths, crews will be worried about triggering another explosion because of the volatile mix of oil, gas and water.

BP faces an equally daunting challenge to contain the political and financial fallout. The Obama administration has kept up the pressure on the oil giant, a move seen in part as a tactic to divert criticism of its own role in the disaster.

Recent news reports have suggested the interior department exercised lax oversight in approving BP's operations in the Gulf, accepting too readily its claims there was little risk of an accident.

The department was further embarrassed by an ABC television report disclosing that Tom Strickland, who is in charge of fish and wildlife, went on a rafting trip in the Grand Canyon with his wife and other officials five days after the spill.