Boeing has told the airline it expects the 737 MAX to be flying by end-September, Ryanair Chief Executive Michael O'Leary told Reuters. He added that while nobody knew when the MAX would be back in service, he believed this target could slip to end-year.

"Boeing are telling us at the moment they expect to be back flying by the end of September," O'Leary said in an interview on the sidelines of an airlines meeting.

"I think it will fly before the end of this year. I am not sure they will meet the end of September date, but I take comfort from the fact that it seems that now the American, European, Brazilian and Canadian regulators are working together," he said.

Boeing's top-selling jet has been grounded in the wake of deadly crashes in Ethiopia and Indonesia in the span of five months, and the US planemaker has been working through a software fix for the jet while facing probes from regulators and US lawmakers as well as lawsuits.

Ryanair is waiting to take delivery of 50 new MAX aircraft, which it can receive at a maximum rate of six to eight per month, he said.

Europe's largest budget carrier is also putting "much more pressure" on Boeing after British Airways owner IAG signed a surprise letter of intent for 200 MAX aircraft last month, O'Leary said.

Ryanair has a deal with Boeing guaranteeing lower prices than its competitors, industry analysts say.