The U.S. Federal Aviation Administration has identified a new potential risk that Boeing Co must address on its 737 MAX before the grounded jet can return to service, the agency told Reuters on Wednesday.
The risk was discovered during a simulator test last week, sources with knowledge of the matter told Reuters.
The new issue means Boeing will not conduct a certification test flight until July 8 at the earliest, the sources said, and the FAA will spend at least two to three weeks reviewing the results before deciding whether to return the plane to service.
Last month, FAA representatives told members of the aviation industry that approval of the 737 MAX jets could happen as early as late June.
The world's largest planemaker has been working on the upgrade for a stall-prevention system known as MCAS since a Lion Air crash in Indonesia in October, when pilots were believed to have lost a tug of war with software that repeatedly pushed the nose down.
Boeing's top-selling 737 MAX was grounded worldwide after a second deadly crash in March in Ethiopia which also involved MCAS. The two accidents combined killed 346 people.