TOKYO — The Japanese and American authorities have lifted the final hurdles to the Boeing 787’s return to commercial flights, more than three months after the plane’s battery problems grounded the fleet.

The Japanese authorities on Friday formally approved Boeing’s fixes to the lithium-ion batteries on its 787 Dreamliner jets and declared the aircraft fit for use. But as an extra precaution before returning the planes to service, Japan’s Ministry of Transport asked All Nippon Airways and Japan Airlines, which together own about half of the 50 Dreamliners delivered so far, to adopt additional safety measures than those required by the American aviation authorities. Those measures include adding monitors to the batteries to read voltage levels in real time.

Separately, the Federal Aviation Administration on Thursday published a new rule that formally lifted the grounding of the 787s operated by carriers based in the United States once the modified lithium-ion batteries are installed. Last week, the F.A.A. had given a green light to Boeing to start modifying the batteries on the 787s. In its new rule, known as an airworthiness directive, the F.A.A. gave its first estimate of the cost of the repairs — $464,763 per plane — though the agency said that could be covered by the plane’s warranty and, therefore, paid by Boeing.

Boeing’s new battery plans are meant to add safety features to the 787’s lithium-ion batteries that will minimize the chances of their emitting smoke or catching fire, after two units overheated on separate planes in January.