Another leak has been found in the flow line beneath the Gannet Alpha oil platform, 113 miles (180km) off Aberdeen, BBC reports.
Shell has been dealing with the release of an estimated 216 tonnes - 1,300 barrels - from a leak near the platform discovered last week.
The Department of Energy and Climate Change said it was a "substantial" spill, but should disperse naturally.
The oil company said it was working to locate the second leak.
Glen Cayley, technical director of Shell's exploration and production activities in Europe, said: "We've got a very complex sub-sea infrastructure and the position of the leak is in an awkward place with a lot of marine growth.
"It's taken our diving crews some time to establish exactly and precisely where that leak is coming from."
On Monday afternoon, Shell said that the sheen from the "light crude oil with a low wax content" was 0.5 square kilometres in size.
A DECC spokesman said: "Shell has informed us that the oil leak at its pipeline at the Gannet Alpha platform east of Aberdeen is under control and has now been greatly reduced.
"They are working to completely halt any further leakage.
"DECC's environmental inspectors will continue to monitor the situation and have been working closely with the company and counterparts from the Health and Safety Executive, Maritime and Coastguard Agency and Marine Scotland since the spill was reported last week."
Shell has been dealing with the release of an estimated 216 tonnes - 1,300 barrels - from a leak near the platform discovered last week.
The Department of Energy and Climate Change said it was a "substantial" spill, but should disperse naturally.
The oil company said it was working to locate the second leak.
Glen Cayley, technical director of Shell's exploration and production activities in Europe, said: "We've got a very complex sub-sea infrastructure and the position of the leak is in an awkward place with a lot of marine growth.
"It's taken our diving crews some time to establish exactly and precisely where that leak is coming from."
On Monday afternoon, Shell said that the sheen from the "light crude oil with a low wax content" was 0.5 square kilometres in size.
A DECC spokesman said: "Shell has informed us that the oil leak at its pipeline at the Gannet Alpha platform east of Aberdeen is under control and has now been greatly reduced.
"They are working to completely halt any further leakage.
"DECC's environmental inspectors will continue to monitor the situation and have been working closely with the company and counterparts from the Health and Safety Executive, Maritime and Coastguard Agency and Marine Scotland since the spill was reported last week."