US authorities have found a new risk on the Boeing 737 MAX aircraft, which could keep the plane grounded for longer.
The US Federal Aviation Administration (FAA) has not elaborated on the issue that was identified.
It said the risk was discovered during a simulator test last week and that it was not yet clear if the issue can be addressed with a software upgrade or will require a more complex hardware fix, according to sources with knowledge of the matter who spoke to Reuters.
The sources said the carrier will not conduct a certification test flight until July 8 in a best-case scenario, with one source saying it could face delays beyond that time.
Boeing's best selling model was grounded in March following two fatal crashes in five months.
The FAA said in a Tweet that it would lift Boeing's "prohibition order when we deem it safe to do so".
FAA representatives had told members of the aviation industry last month that approval of the 737 MAX jets could happen as early as late June.
Boeing is upgrading the plane's anti-stall software, which has been the focus of the crash investigators. However, the FAA is also looking into whether the aircraft has a hardware issue.