US regulators have uncovered a possible new flaw in Boeing's troubled 737 Max aircraft that is likely to push back test flights.
The Federal Aviation Administration (FAA) said it had identified the "potential risk" during simulator tests, but did not reveal details.
Boeing's top-selling aircraft was grounded in March after two crashes.
The company is upgrading the aircraft's flight control system, which is the focus of crash investigators.
The control system can help prevent a plane from stalling.
In a tweet, the FAA said: "On the most recent issue, the FAA's process is designed to discover and highlight potential risks. The FAA recently found a potential risk that Boeing must mitigate."
A source familiar with the situation told the BBC: ""During simulator testing last week at Boeing, FAA test pilots discovered an issue that affected their ability to quickly and easily follow the required recovery procedures for runaway stabiliser trim (ie, to stop stabilisers on the aircraft's tail moving uncontrollably).
"The issue was traced to how data is being processed by the flight computer."
Last month, the FAA indicated that approval of Boeing's changes to the 737 Max could come in late June. That would have allowed test flights in early July.
There were initial hopes among airlines that the 737 Max would be back in the air during the summer, but that timetable was pushed back to late this year even before the latest news.
