Boeing has said it does not expect its 737 Max plane to return to the skies before the summer, which is longer than initially expected.
The jet has been grounded since March after two fatal crashes, which together killed 346 people.
Boeing has said it is working on a software update intended to fix the problem blamed for the crashes.
But the company has struggled to convince regulators that the planes are safe to fly.
It previously said it expected the planes to be cleared for approval before the end of 2019. But in December, Boeing announced plans to halt production of the aircraft and last week, it confirmed it had found a new problem in the software.
‘Number one priority’
Boeing shares dropped by more than 5% on Tuesday as word of the delay started to spread, prompting the New York Stock Exchange to temporarily halt trading until the company’s formal update.
Boeing said its new guidance was “informed by our experience to date with the certification process”.
“Returning the Max safely to service is our number one priority, and we are confident that will happen.”
The grounding has already cost Boeing more than $9bn, and the consequences are starting to spread, with at least one major supplier announcing layoffs.
29 October 2018: A 737 Max 8 operated by Lion Air crashes after leaving Indonesia, killing all 189 people on board
31 January 2019: Boeing reports an order of 5,011 Max planes from 79 customers
10 March 2019: A 737 Max 8 operated by Ethiopian Airlines crashes, killing all 157 people on board
14 March 2019: Boeing grounds entire 737 Max aircraft fleet