As BP PLC defended its handling of the oil spill in the Gulf of Mexico, documents show it argued against new, stricter safety rules proposed last year by the U.S. agency that oversees offshore drilling.

The British oil giant was one of several companies that wrote to the U.S. Minerals Management Service this past September saying additional regulation of the oil industry was unnecessary. In a letter, BP said the current voluntary system of safety procedures was adequate.

BP...