An oil spill in the North Sea is estimated to amount to several hundred tonnes, making it the biggest such leak in more than a decade, according to UK government figures.
The spill – far greater than annual totals dating back to 2001 – was described as "substantial" by the Department of Energy and Climate Change (Decc).
The total amount of oil discharged into the North Sea in 2009 was 50.93 tonnes.
Shell, which operates the Gannet Alpha platform about 112 miles east of Aberdeen, said the subsea well was shut on Wednesday, but it has not confirmed the quantity of the leak.
A Decc spokesman said the energy firm is still trying to "completely halt" any further leakage.
The spokesman said: "Although small in comparison to the Macondo, Gulf of Mexico, incident, in the context of the UK continental shelf, the spill is substantial – but it is not anticipated that oil will reach the shore and indeed it is expected that it will be dispersed naturally.