Royal Dutch Shell estimated Monday that 54,600 gallons of oil have spilled into the North Sea from an oil rig off Scotland's eastern coast.
The Gannet Alpha oil rig, located 112 miles (180 kilometers) east of the city of Aberdeen, is operated by Shell and co-owned by Shell and Esso, a subsidiary of the U.S. oil firm Exxon Mobil.
Glen Cayley, technical director of Shell's European exploration and production activities, called the spill "significant" given the amount of oil that generally spills into the North Sea.
"We care about the environment and we regret that the spill happened," he said.
Cayley said he believed waves would disperse the oil sheen and the spill was not expected to reach the shore.
It was not clear when the leak began last week. Shell announced it Friday and said it was under control on Saturday.
Cayley said the flow line to the Gannet Alpha platform was now leaking around five barrels a day. He also said there was some hydraulic fluid in the spill but all the people on the oil rig were safe and the platform was still operating.
The British government said the leak was small compared to the BP spill in the Gulf of Mexico last year _ which dumped 206 million gallons of oil into the Gulf _ but said it was still substantial for the U.K.'s continental shelf. It backed up Shell's predictions that the oil would disperse naturally.
It said Britain's offshore oil industry had a strong safety record "which is why it is disappointing that this spill has happened. We take any spill very seriously and we will be investigating the causes of the spill and learning any lessons from the response to it."
The government said the Maritime and Coastguard Agency, which monitors the waters around Britain, was making twice-daily flights over the area to monitor the situation.
Shell said in a weekend statement the spill covered an area 19 miles wide by 2.7 miles long (31 kilometers by 4.3 kilometers).
There are several small leaks into the North Sea each year. The U.K. Department of Energy and Climate Change estimates that in 2009, around 51 metric tons (56 short tons) of oil was released into the sea. The current spill is about four times that amount _ 216 metric tons (238 short tons).
The last major incident in the North Sea was in 1993, when the MV Braer carrying 85,000 metric tons (93,700 short tons) of crude oil ran aground in a storm in the Shetland Islands.
The British government has already beefed up its inspections of the 24 drilling rigs and 280 oil and gas installations in Britain's part of the North Sea in the wake of the 2010 Gulf spill.
Stuart Housden, Director of the Royal Society for the Protection of Birds Scotland, said razorbills, puffins and guillemots that gather in the North Sea in late summer could be at risk.
"We know oil of any amount, if in the wrong place at the wrong time, can have a devastating impact on marine life," he said.
The Scottish government said it was working with Shell to monitor the spill and warn local fishing boats about it.
McConville reported from Edinburgh
(This version CORRECTS number of gallons in headlines.)