Ryanair may not receive any 737 MAX aircraft from Boeing by next summer over European delays in testing the grounded jets, group chief executive Michael O'Leary said.
Mr O'Leary estimated that the issue is costing the airline at least €100m a year.
Ryanair trimmed its passenger traffic forecast last week but that was still based on 10 MAX deliveries by June.
Mr O'Leary told Reuters today that testing in Europe was running behind the US and planes were likely to remain grounded until April or May.
"We're still looking to meet with Boeing in mid-early January. We expect the MAX to be back flying in early January, particularly in North America," Michael O'Leary said in an interview.
"We think it could be a bit slower in Europe because the EASA (European Union Aviation Safety Agency) seems to be dragging their heels a little bit," he added.