A Massachusetts congressman is asking BP to accept the government's latest estimate of how much oil spewed from the company's blown-out Gulf well as a basis to determine how much it may face in civil fines.

Rep. Ed Markey sent a letter Wednesday to the head of BP's U.S. operations telling him that the oil giant should legally own up to its obligations as one of the responsible parties for the spill.

Civil penalties can be levied under a variety of environmental protection laws, including fines of up to $1,100 for each barrel of oil spilled, more if BP is found to have committed gross negligence or willful misconduct.

Based on the roughly 4.9 million barrels, or 206 million gallons, of oil that government experts are now saying spilled from the well, BP could face fines of up to $21 billion.