A possible new flaw in Boeing’s 737 Max aircraft could push back test flights.
The FAA said it has identified a ‘potential risk’ during simulator tests, but did not disclose details.
It tweeted: "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."
Boeing said it was working closely with the FAA and believed a software fix would address the problem.
The 737 MAX aircraft was grounded in March following two fatal crashes.
It was originally believed tests flights could start next month after changes to the aircraft’s flight control system were approved.
But this new setback is likely to cause additional delays.
At this week’s ABTA Travel Matter Conference, Martin Alcock, director of The Travel Trade Consultancy, said the grounding of the 737 MAX was a major problem for TUI.
"In a difficult trading environment, TUI is also having to spend money on other aircraft. It’s been a really challenging time," he said.
TUI has 15 737 MAX for the UK, Belgium, the Netherlands and Sweden and a further eight were due to be delivered last month.
Earlier this month British Airways parent IAG signed a letter of intent for 200 737 MAX aircraft.
Subject to formal agreement, the mix of 737 MAX 8 and 737 MAX 10 aircraft are due to be delivered between 2023 and 2027, and would be operated by BA, Vueling and Level.
It is believed IAG got the aircraft at a substantial discount.
IATA director general Alexandre de Juniac said this week: "The Boeing 737 Max tragedies weigh heavily on an industry that holds safety as its top priority.
"We trust the Federal Aviation Administration, in its role as the certifying regulator, to ensure the aircraft’s safe return to service. And we respect the duty of regulators around the world to make independent decisions on FAA’s recommendations.
"At the same time, aviation is a globally integrated system that relies on global standards, including mutual recognition, trust, and reciprocity among safety regulators.
"Aviation cannot function efficiently without this co-ordinated effort, and restoring public confidence demands it."
