Royal Dutch Shell PLC said Friday it has closed a valve that was seeping oil from a pipeline off Scotland's coast, stopping the worst spill in a decade for Britain's North Sea oil fields.
The company said closing the valve was a "key step" in halting the leak at its Gannet Alpha platform, but that it would monitor the pipeline to make sure the valve remains sealed.
The leaking valve is on a "flow line" that carries oil away from the well on the ocean floor.
Divers sealed off the valve, 300 feet (90 meters) under water, in a "careful and complex operation," said Glen Cayley, technical director of Shell's exploration and production activities in Europe.
"But we will be watching the line closely over the next 24 hours and beyond," he said.
"Our next task is to remove the residual oil from inside the de-pressurized flow line, and that will take time." Shell has said the pipeline still contains up to 660 tons, or 4,620 barrels, of crude oil.
About 1,300 barrels of oil has spewed into the sea since a pipeline was found to be leaking Aug. 10, the company said, although it claimed that after it shut the well, only one barrel a day had been leaking from a relief valve to control the build-up of pressure in the pipeline.
The spill, 110 miles (180 kilometers) east of Aberdeen, Scotland, has caused a sheen covering an area of 2.5 square miles (6.7 square kilometers), authorities said.
Secretary of State for Scotland Michael Moore said closing the valve was "clearly positive news," but that "no oil leakage into our waters is acceptable and lessons must be learned." He promised officials would conduct "a thorough investigation."
Environmental groups have expressed alarm at the potential impact of the spill on wildlife, and have criticized Shell for a lack of transparency about the extent of the pollution.
The leak was discovered Aug. 10, but Shell only made it public two days later. On Saturday, Shell said the spill had been brought "under control," but it later emerged a small amount of oil was still leaking into the sea.
The Royal Society for the Protection of Birds urged Shell to involve the Scottish government and environmental groups in the British government's investigation of the spill, warning that puffins, razorbills and guillemots on the open seas were at risk.
But the oil company said surveys of bird and marine life "have indicated no significant impact on the environment." It says the oil will disperse at sea and not reach the shore.
The company said it had set up an investigation team to establish the cause of the leak and was cooperating with government inquiries into the spill.
The last major spill in the North Sea occurred in 1993, when the oil tanker MV Braer ran aground in the Shetland Islands, spilling around 620,000 barrels of crude into the sea.