Ryanair boss Michael O'Leary has warned more strikes may hit peak summer holiday season as the airline's profits slumped by 20pc.
The chief of the low-cost carrier also took a hard line with some unions, threatening job losses if they keep up their "unreasonable demands".
Shares in the airline tumbled more than 4pc after it emerged its profits had been stung by lower fares, higher oil prices and a pay rise for pilots.
But Ryanair is also currently grappling with a series of strikes that is disrupting peak summer holiday travel.
Sources have revealed Irish-based pilots are set to announce more strike dates tomorrow. A Fórsa disputes committee is meeting to consider ramping up industrial action.
Up to 2,500 passengers will be hit today as 16 flights are cancelled during a third strike by the pilots who are demanding better terms and conditions.
Some 100,000 passengers across Europe also face disruption as 600 flights are grounded due to cabin crew strikes tomorrow and Thursday.
"We expect further strikes over the peak summer period as we are not prepared to concede to unreasonable demands that will compromise either our low fares or our highly efficient model," said Mr O'Leary.
He said if the "unnecessary" strikes continue to damage customer confidence and "yields" in some markets, the airline would have to review its winter schedule.
This could lead to a reduction in the fleet and "job losses in markets where competitor employees are interfering in our negotiations with our people and their unions".
"We cannot allow our customers flights to be unnecessarily disrupted by a tiny minority of pilots," he added.
Ryanair has blamed some Aer Lingus pilots for steering the campaign of industrial action by its directly-employed pilots in Ireland.
Mr O'Leary said Ryanair profits were hit by higher costs and a fear of a no-deal Brexit.
Average fares are expected to be lower over the summer due to the World Cup, the heatwave across northern Europe, and uncertainly about pilot strikes, Ryanair said.
Pre-tax profits for the three months to the end of June fell to €319m from €397m a year earlier. However, full-year profit forecasts remain unchanged at between €1.25bn and €1.35bn, although the prediction depends on stable fares and no Brexit shocks.
Fórsa hit back at Mr O'Leary's jobs claims and said it told the airline it was available for talks last Friday.
"This kind of threatening statement is not conducive to building trust and reaching a resolution to the dispute, and Fórsa doesn't accept that jobs or expansion in the airline need be put at risk by company management," said spokesperson Bernard Harbor.
"The pilots' demands are reasonable, and would not add any significant costs or undermine the airline's business model."
He said the pilots want a "fair and transparent" method to decide base transfers and other matters, which is common practice in the industry.
The Ryanair Transnational Pilot Group praised Irish pilots for taking an "enormous and historic step".
"Their right is our fight," it said in a letter.