Ryanair Holdings PLC’s chief executive said Tuesday the Irish budget airline was unlikely to place a big order for new aircraft with Boeing Co., a move that could harm the airline’s prospects for future growth.

“Sadly, the deal is now highly unlikely to happen,” Chief Executive Michael O’Leary said in an interview.

Ryanair is one of Boeing’s biggest airline customers. It has been negotiating with Boeing for more than a year on the terms of an order for 200 of its 737 model.

“We effectively reached agreement on the price for 200 aircraft, but Boeing now wants to renegotiate delivery terms that would be worse than we currently have,” Mr. O’Leary said, adding that talks had broken down.

Ryanair’s board will meet Dec. 17. Mr. O’Leary said that if no deal had been reached by then, the deal would be off. The CEO recently complained that Boeing’s pace of negotiations was “glacial.” A Boeing spokesman said the company wouldn’t comment on customer negotiations that were ongoing.

If the airline and Boeing can’t strike a deal, Mr. O’Leary has said that Ryanair will throttle back its expansion plans. It will defer deliveries from Boeing of several 737s ordered in recent years, cancel options to buy 10 737s, and distribute cash to shareholders. Beyond 2012, the company would largely stop growing for the foreseeable future, Mr. O’Leary has said.