BP intends to challenge official government estimates of how much oil leaked from its runaway well in the Gulf of Mexico, a move that could reduce its legal liability by billions of dollars, according to documents the company filed with the presidential commission investigating the spill.

In August, federal scientists estimated that 4.9 million barrels of oil had leaked from the well before it was capped on July 15, a figure that BP now argues is 20 to 50 percent too high. Under the Clean Water Act, BP faces fines of up to $21 billion, or $4,300 per barrel, if courts determine that it acted with gross negligence before the accident.

BP has not offered its own estimates of how much oil spilled, but in the documents filed with the National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, it questioned the accuracy of the government’s figures, Priya Aiyar, the panel’s deputy chief counsel, said at its final hearing on Friday.

In a letter to the commission in October, BP argued that official estimates by the United States Geological Survey and the Energy Department were “flawed.”