BP lawyers have said that government estimates of the size of the Gulf of Mexico oil spill are too big, perhaps by as much as 20 to 50 percent, signaling a dispute that will determine how many billions of dollars of fines BP will have to pay under the Clean Water Act.

The BP attorneys made their comments in an Oct. 21 meeting with staff members of the National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, commission lawyer Priya Aiyar revealed at a hearing Friday.

BP also took issue with one of the presidential oil spill commission’s draft reports. BP said that the assertion that “a consensus is emerging that roughly 5 million barrels of oil were released by the Macondo well is both premature and inaccurate.”

BP said that the estimates by the Energy Department and an inter-agency group “appear biased toward the maximum amount of oil that could have been discharged, rather than the amount of oil most likely to have been discharged.”