WASHINGTON — The Federal Aviation Administration has accepted Boeing’s revamped battery system for its beleaguered 787 Dreamliners and agreed to lift its grounding order, according to a congressional official today.

The order gives Boeing the go ahead to begin retrofitting planes with an enhanced lithium ion battery system although the root cause of battery failures that caused a fire on one of the planes and smoke on another is still unknown.

Boeing intends to work on the retrofits over the weekend, and flights could resume within days to a week, the official said.

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.

The FAA’s action directly affects United Airlines, which is the only U.S. airline with 787s in its fleet. But aviation authorities in other countries are expected to swiftly follow suit.