BP has said it may be next spring before it can perform the test, which involves sending a cleaning device called a "pig" equipped with sensors through the pipeline.

The PHMSA said in its ruling BP will be permitted to operate the lines while it comes up with a plan to carry out the smart pigging test.

BP officials say they may not be able to test the lines until next spring due to the amount of sediment in the lines. Federal investigators carrying out a criminal probe into the spill also have ordered BP to preserve the pipeline that leaked in its current state.

In a letter dated June 15, Rep. John Dingell, the ranking Democrat on the House Energy and Commerce Committee, asked the PHMSA to clarify how long the government is willing to allow BP to continue to operate the transit lines in the absence of a smart pigging test given regulators' own opinion that smart pigging was the best corrosion detection method available.

The PHMSA is expected to reply to the letter by the end of the month.

BP chief executive John Browne defended BP's pipeline maintenance practices in Alaska in an interview with Reuters last week, saying that "world-class" techniques were applied to detect corrosion.

Browne also noted that BP's corrosion management strategy had the approval of Alaskan environmental authorities, who, prior to the spill, regulated BP's oil transit pipelines. Since the spill, BP officials say the company has increased its corrosion detection efforts.