When BP suggested over the weekend that it might leave the Gulf of Mexico well capped until it completed the relief wells, Representative Edward J. Markey, Democrat of Massachusetts, suggested that the company might have an ulterior motive  to limit its own spiraling legal liability.

Mr. Markey said reopening the well would allow the first accurate measure of the flow and accused the company of trying to forestall a reckoning of the spill.

Until now, the actual flow rate of oil gushing into the gulf has been based on estimates. With the well tightly capped, any efforts to collect the oil to ships on the surface would provide a better reading of the oil flow. BP’s ultimate liability for damages from the spill will be based in part on that flow.

“BP has a stake in lowering its liability,” Mr. Markey said in an interview, since fines are assessed by the barrel. The company, he said, has consistently underestimated the flow out of its runaway well.