The Obama administration granted Royal Dutch Shell final clearance on Monday to resume drilling for oil and gas in the environmentally fragile Arctic Ocean for the first time since 2012, a move green groups vowed to fight.
The US Department of the Interior permit allows Shell to drill in the oil-rich Chukchi Sea off the northwest coast of Alaska. Shell interrupted its drilling programme in the region in 2012 after suffering a series of mishaps, including losing control of an enormous rig, from which the Coast Guard had to rescue 18 workers.
Harsh conditions in the Chukchi have discouraged other oil companies from drilling there.
The go-ahead for Shell comes after repairs were completed to the Fennica, an icebreaker the company leases that carries emergency well-plugging equipment. The ship had suffered a gash in its hull after hitting uncharted shoals off southern Alaska.
Damage to the Fennica had stalled Shell's programme, which the Interior Department had previously issued a permit for.
Shell obtained the leases in the Chukchi during the administration of former President George W. Bush. Since then it has spent about $7 billion on exploration in the Arctic, though oil production is at least a decade away.
The Arctic is home to what the US government estimates is 20 per cent of the world's undiscovered oil and gas.
Shell's determination to drill there has spawned waves of protests and funding drives by environmentalists who want to protect whales, walruses and polar bears in a vulnerable region that scientists say is changing rapidly due to global warming.
Late last month, 13 Greenpeace activists hanging from a bridge in Oregon temporarily blocked the freshly repaired Fennica from reaching the Pacific Ocean to return to Alaska.
President Barack Obama "must change the course on Arctic drilling set eight years ago by former President George W. Bush and not perpetuate it," said Michael Brune, head of the Sierra Club, the country's oldest environmental group.
The club urged Obama to cancel sales of oil-zone leases scheduled for 2016 and 2017 and to remove the possibility of drilling in the Arctic Ocean.
Later this month, Obama will visit Alaska to speak at a conference on the Arctic and tour areas threatened by climate change.
Curtis Smith, a Shell spokesman, said the company "looks forward to evaluating what could potentially become a national energy resource base."
Shell is not releasing a timetable for its drilling programme.