* Largest Gulf operator trails other firms filing * BP was faulted by investigators in spill disaster * Plan is for Kaskida discovery in Keathley Canyon * BP's 2006 find highly touted after 2009 appraisal well By Bruce Nichols HOUSTON, Sept 23 (Reuters) - BP Plc. confirmed on Friday it has filed a plan with U.S. regulators to pursue its first new deepwater oil exploration work in the Gulf of Mexico since the disastrous Macondo spill in 2010. The plan, filed with the Bureau of Ocean Energy Management, Regulation and Enforcement, is BP's first move to return to deepwater Gulf exploration since its Macondo well ruptured on April 20, 2010, and caused nearly 5 million barrels of oil to spew into the sea in what was the worst U.S. marine oil spill. The supplemental exploration plan will be a key litmus test for how U.S. regulators treat BP, the biggest Gulf oil producer, in the post-Macondo world. In a key investigative report issued on Sept. 14, the U.S. government heaped the lion's share of blame for the disaster on BP, which faces a raft of criminal and civil litigation and billions of dollars in potential damages. BP's filing calls for further appraisal drilling at the 2006 Kaskida discovery in the Keathley Canyon area. An early appraisal well in 2009 confirmed oil in the highly touted Lower Tertiary play in the Gulf. "We have filed a supplemental exploration plan," a BP spokesman confirmed. "Obviously, we had an exploration plan and we've added to it." The offshore Gulf is key to the future of BP and U.S. domestic oil supply. BP produces about a quarter of the oil and natural gas output from the region that generates about a third of total U.S. domestic production. After calling a full-stop on new offshore drilling after the spill, U.S. regulators have approved new drilling plans for many other companies, including Royal Dutch Shell and Chevron , but not for BP. "I think it will be interesting to see whether BP is treated any differently. The word is they won't, but we'll see," said Phil Weiss of Argus Research in New York. "It's significant in that, assuming it's approved, it gives BP the ability to get back to work on a project they're in charge of," Weiss said. BOEM confirmed BP's application is its first since Macondo and said BP will receive standard treatment. "All operators are held to the same enhanced safety and environmental standards put in place following the Deepwater Horizon explosion and oil spill," a BOEM spokeswoman said. BP said it could not predict when drilling would begin until the application is approved. But the application specifies a target date of Nov. 1 for the first of four wells to be drilled through 2013. BP estimates each will take 205 days to drill. "We know there's a period of time BOEM ascribes to this process, but the exact timing of activity depends on the approval of the regulator," the BP spokesman said. The proposed new drilling is 290 miles (466 km) southwest of the Macondo well site. It is in 5,800 feet of water, about 800 feet deeper than Macondo.