Shell said today it is working to stop a secondary flow of oil spilling into the North Sea from beneath an offshore platform.
An initial leak was brought "under control" last week but work is continuing on a smaller leak from the same source in an area surrounded by marine growth.
About 216 tonnes of oil - equal to 1,300 barrels - were estimated to have spilled from the Gannet Alpha platform, 112 miles east of Aberdeen, by yesterday.
The amount is far greater than the annual totals for the North Sea over the past decade, according to figures from the Department of Energy and Climate Change.
The total amount of oil discharged into the North Sea in 2009 was 50.93 tonnes.
Environmental groups have heavily criticised the operator for the way it has handled the leak, which was first detected on Wednesday.
Shell said the current rate of leakage is less than five barrels a day.
A Shell spokeswoman said: "The leak source remains the same. The initial release path was stopped on Thursday, however the oil found a second pathway to the sea.
"Since then we have been working to find the source of the much smaller flow of hydrocarbons.
"It had proved difficult to find because we are dealing with a complex subsea infrastructure and the position of the small leak is in an awkward place surrounded by marine growth.
"So it has taken our ROV inspections some time to establish exactly where the source is.
"We believe now that the flow is coming from a relief valve adjacent to the original leak and from the same source.
"Once we've confirmed this we will then develop a series of mitigation options to stop this leak. There is no new leak."
The spill is described as a light crude oil with a low wax content. Hydraulic fluid is also present.
At its peak the sheen on the surface extended 18 miles, but Shell said this has now diminished.
Dr Richard Dixon, director of wildlife organisation WWF Scotland, said: "It is clear that Shell are having great difficulty dealing with their leaking pipeline.
"It really does make you question the entire oil industry's ability to respond had this accident been on a larger scale or in the much more difficult waters of the Arctic."
Per Fischer, of Friends of the Earth Scotland, said: "It beggars belief that we are still being drip-fed information and that Shell's initially 'insignificant' leak is still causing problems."
Greenpeace senior oil campaigner Vicky Wyatt said: "While oil has been flowing, timely information has not. The original leak, now classified as significant, happened on Wednesday, but the news wasn't made public for 48 hours, and now we're learning of a second spill.
"As Shell finalises plans to move into the fragile Arctic, where oil spills are almost impossible to clean up, the company has important questions to answer. Meanwhile the Government should halt its rush to hand out new licences for deep water drilling to the west of Shetland."
Scottish Environment Secretary Richard Lochhead said: "In the coming days Marine Scotland's fishery research vessel Scotia will take fish, seawater and sediment samples to monitor any environmental impact the leak may have caused.
"There are currently no Scottish fishing vessels operating close to the vicinity of the oil leak, therefore no impact is expected in terms of contaminated fish entering the food chain, but we remain vigilant.
"Our understanding is that output from the North Sea oil leak has been greatly reduced, and that Shell is continuing work to stop the flow completely. It is important that Shell are as open and transparent as possible, and provide regular updates on the developing situation."
He said the administration must be given a full and formal role in any UK investigation.
Labour's environment spokeswoman at Holyrood, Sarah Boyack, said: "This is the most serious oil spillage in a decade and it is important that everything surrounding this accident is transparent and the oil firm do not decide that they'll only release information on their terms.
"Oil is a major Scottish industry and it's important all safeguards are in place to protect human and marine life."
She called for the Scottish Government to give details of its discussions with Shell, and for details of contingency plans from Marine Scotland.
Tom Brock, chief executive of the Scottish Seabird Centre, said he is concerned about the spill and requested information from Shell.
He said: "Over the last few months we have been watching amazing pictures of seabirds including puffins, guillemots, razorbills and kittiwakes live on our remote cameras successfully rearing their chicks.
"We are now very concerned that these birds could be at serious risk if they come into contact with the spillage.
"We are well positioned and willing to provide advice, but cannot do this without Shell confirming the full details of the situation."
Dr Simon Boxall, an oceanographer at the University of Southampton, said the spill amounts to about 10% of the platform's daily production over the past four to five days.
He assessed that the crude oil will break down "relatively quickly" and the chance of it reaching land is "slight".
But he added: "Any spill is harmful to the environment in some way and whilst this is not a major incident it will cause some damage.
"More critically in the cooler waters of the North Sea microbial action takes longer and so the oil remains harmful for longer as well."
He said attempting this type of operation in poorer winter weather would be "dangerous and ill-advised".
Dr Boxall continued: "Royal Dutch Shell, the operators, and their co-operators on the platform Esso, are amongst a number of companies keen to extract oil from deeper sources to the north-west of Shetland.
"This oil is needed if north-west Europe is to maintain its own oil and gas supplies, but it carries the dangers of deep water in a cold and stormy climate - the worst of both worlds.
"After BP's experience in the Gulf of Mexico, and in spite of it, many other oil companies were reticent to see significant changes to safe practices in other parts of the World.
"Whilst this is an unusual event - the North Sea platforms do have a relatively good safety record, it does highlight the problems ahead."