A Ryanair cabin crew awaits as passengers disembark on the tarmac at Brussels South Charleroi Airport in Charleroi on September 27, 2018.

In 1995, soon after taking over as chief executive of Ryanair, Michael O'Leary wrote a characteristically plain-speaking memo. His mission to control costs would be "ruthless . . . at the expense of charm, style and elegance if necessary".

Mr. O'Leary has been true to his word: Ryanair's hard-nosed culture has produced an airline with few frills but market-leading profitability and a superior valuation.

But the low-cost carrier, which issued a profit warning this week and its shares dropped 11 percent in response, is suffering its most turbulent period, beset by strikes, high fuel prices and fiercer competition.

Investors and industry figures are even asking whether Ryanair's best days are behind it.

Daniel Roeska, an analyst at Bernstein, reckons that last year's net income of €1.45bn "is an earning level they will not be reaching any time ever again soon". This week's warning brought the 2019 profit target down more than 10 percent to €1.1bn-€1.2bn.

In an interview with the Financial Times, Mr. O'Leary warned that there might be further bad news to come: "Do we have to trim guidance again for the year? We hope we don't have to but again it can't be ruled out if [ticket] pricing continues to fall and oil prices continue to rise."

He was sanguine, though, about the company's stock: "Our share price has been grim, but the performance of most of our peers has been equally grim."

It is true that most airlines have suffered this year but Ryanair, whose shares have fallen by almost 30 percent, has previously outshone the market. On a five-year basis, IAG, the parent of British Airways and Iberia, has now caught up with its low-cost rival, having spent most of that period trailing by a large margin.

While the industry has been hit by a surging oil price and over capacity, Ryanair has endured a particularly painful series of strikes that have left passengers exasperated and investors worried that Mr. O'Leary is losing his ability to control costs.

"The employment practices of Ryanair were unique and — as it appears now — not sustainable," said Andrew Lobbenberg, an analyst at HSBC. "Their units costs will still be very good, and almost certainly among the best in the industry, but the gap will narrow."