BP is mounting a new challenge to U.S. government estimates of how much oil flowed from the runaway well deep below the Gulf of Mexico. The issue will be critical in determining the size of federal pollution fines the company will pay.

Staff working for the presidential oil spill commission told The Associated Press that BP is arguing that the government overestimated by as much as 50 percent the amount of oil that spilled. The company's argument could reduce BP's likely fines under the Clean Water Act by as much as $2.7 billion.

Money collected from BP could be used to pay for restoration of the Gulf coast.

The federal government has estimated that about 2.6 million gallons of oil a day spewed from the well, declining to about 2.2 million gallons daily before the well was capped in mid-July.