Ahead of a meeting of more than 30 international air regulators on Thursday (23 May), Dan Elwell said he was “not tied to a timetable” for the aircraft’s return, Reuters reports.

Elwell is expected to brief representatives on its latest safety analysis, Boeing’s proposed software update for the 737 Max and any new training requirements.

“If it takes a year to find everything we need to give us the confidence to lift the [grounding] order so be it," he told reporters ahead of the meeting.

The 737 Max was grounded in March following a second fatal crash involving the aircraft in five months.

Last October, Lion Air Flight 610 plunged into the Java Sea shortly after take off from Jakarta killing all 189 people on board.

Then in March, Ethiopian Airlines flight 302 came down en route to Nairobi from Addis Ababa. All 157 people on board died in the crash.

Preliminary investigations into the two incidents suggest the same control system – the aircraft’s manoeuvering characteristics augmentation system (MCAS) – is likely to have been active during both crashes.

Boeing has been working on a software update for the 737 Max but this will require approval and certification from the FAA before it can be rolled out to the nearly 400 737 Max aircraft that were in service before the Ethiopian Airlines crash.

Another 4,700 737 Maxs are understood to be on order across the world. However, many of these have now been delayed and a small number cancelled. Ryanair has confirmed delivery of its proposed new 737 Max 200 fleet has been delayed.

Tui has 15 737 Maxs and Norwegian 18, with a handful operated by other European airlines. The majority though are concentrated in the US and the Far East where they are used on domestic and intercontinental routes.

Tui has warned that should the 737 Max remain out of service until mid-July, it could reduce its profits by around €200 million (17%). If it remains out of service until late September, during the entire summer season, that hit could increase to €300 million (26%).