WASHINGTON -- Federal officials intend to lift the order grounding the beleaguered 787 Dreamliner after accepting Boeing's revamped battery system even though the root cause of battery failures that led to a fire on one plane and smoke on another remains unknown.The Federal Aviation Administration said Friday that it would send airlines instructions and publish a notice next week lifting the 3-month-old grounding order. Boeing will then have the go-ahead to begin retrofitting planes with an enhanced lithium ion battery system.Dreamliner flights could resume within a week, the agency told members of Congress. Boeing said it has stationed teams around the world to begin installing the fix.The FAA gave Boeing permission last month to test the revamped system, which includes additional insulation around each of the battery's eight cells to prevent a short circuit or fire in one of the cells from spreading to the others.The new system also includes enhanced venting of smoke and gas from inside the battery to outside the plane. A strengthened box to hold the battery is an effort to ensure that if a fire were to occur, it wouldn't escape to the rest of the plane.Boeing has completed 20 separate tests of the new system, FAA Administrator Michael Huerta told Congress earlier this week.Boeing had delivered 50 planes to eight airlines in seven countries when a fire erupted in a battery aboard a Japan Airlines 787 parked at Boston's Logan Airport on Jan. 7. The FAA and other authorities grounded the entire fleet after a second incident nine days later led to an emergency landing by an All Nippon Airways 787 in Japan.Boeing said new batteries and kits with the parts for the new battery systems are ready to be shipped immediately. The 787s will get the fix in approximately the order they were delivered, Boeing said.The FAA's action directly affects United Airlines, the only U.S. airline with 787s in its fleet. But aviation authorities in other countries are expected to follow suit swiftly.United Airlines already has domestic 787 flights scheduled for May 31. Spokeswoman Christen David said no other schedule changes have been made yet. Its launch of Denver-to-Tokyo Narita flights is still planned for June 10, but that will depend on installing the battery fix by then, she said.The 787 is Boeing's newest and most technologically advanced plane. It is the world's first airliner made mostly from lightweight composite materials.