Ryanair on Monday said that Boeing had warned it of possible delays to 21 737 MAX aircraft due for delivery before the end of the year and said it was also worried about the certification of the new MAX 10 aircraft.

Boeing remains a "major area of concern" for Ryanair, one of the U.S. aerospace giant's leading customers, Group Chief Executive Michael O'Leary told investors on a call following the release of quarterly results.

Boeing did not immediately respond to an emailed request for comment.

O'Leary said he was confident that Boeing would deliver all 50 737 MAX aircraft due by next summer but he was concerned about a possible repeat of delays this year that forced it to take deliveries during its busy summer season.

"In the last two weeks, we're getting letters out of Boeing telling us there might be problems with 21 aircraft this side of Christmas," O'Leary told investors on a conference call.

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A delay would be "inexplicable and unacceptable," he added.

O'Leary said that he had been assured that Ryanair deliveries would take priority this winter.

"Management in Seattle... need to get their finger out," O'Leary said, repeating a call for a change in management at Boeing's Seattle operations.

O'Leary said he was still interested in a large order for the MAX 10 - the largest member of its best-selling single-aisle airplane family - but that Boeing had not offered attractive enough pricing yet.

"We would hope eventually they will get there," he said.

Ryanair last year walked away from negotiations with Boeing for 200 of the MAX 10.

The MAX 10 programme, which has not yet been certified, received a boost at Britain's Farnborough Airshow last week with Delta Air Lines ordering 100 and Qatar Airways 25.

O'Leary said he was worried about delays to the certification and the possibility that the cockpit would have to be redesigned.

Boeing faces a December deadline to win approval for the 737 MAX 10, otherwise, it must meet new cockpit alerting requirements under a 2020 law, unless waived by Congress.

+Additional reporting Reuters