The United States government says the ruptured BP well in the Gulf of Mexico spilled nearly five billion barrels of oil into the sea before it was capped, making it the biggest accidental spill ever.
Government scientists say only a fifth of the oil was captured during the clean-up operation. The well was temporarily capped two weeks ago.
Oil stopped gushing from the well when a cap was installed on 15 July, enabling US government scientists to produce what may be the definitive estimate of the scale of this disaster, the BBC reports.
Scientists now say a total of 4.9 million barrels of oil leaked from the well, of which 800,000 were captured using various containment methods.
However, that would still make this the biggest accidental oil spill in history - comfortably larger than the spill off the coast of Mexico 31 years ago.