Shares in Easyjet have fallen 7% after the airline said the disruption from April's air traffic control strikes in France would cut profits by £25m.
The carrier said the disruption, which led it to cancel 600 flights, would knock £25m off its pre-tax profits.
The share price fall came despite Easyjet reporting a profit of £7m in the six months to 31 March, compared with losses of £53m last year.
Profits were helped by cheaper fuel and a strong finish to the ski season.
Competitive environment
Airlines tend to incur losses in the winter and make most of their profits during the summer.
Easyjet's half-year profit was at the upper end of forecasts, since it had previously predicted an outcome somewhere between a £5m loss and a £10m profit, while revenues for the six-month period rose 3.8% to £1.767bn.
Total revenue per seat rose by 0.2% to £54.91, while costs per seat fell 1.4% to £38.66.
"The profit in the half reflects the delivery of our customer-focused revenue initiatives and a strong finish to the ski season, as well as the benefit we received from the lower fuel price and favourable foreign exchange movements," said Easyjet chief executive Carolyn McCall.
"As we enter the important summer season, forward bookings are in line with last year, and as we predicted, passengers are benefiting as fares fall to reflect a more competitive operating environment and lower fuel costs."
She told the BBC's Today programme that low fuel prices had been "enormously helpful" in the first half and that foreign exchange movements had gone in Easyjet's favour. However, she warned that currency movements would "go against us in the second half".
The company saw an exchange rate gain of £18m in the first half, but expects that to be reversed in the second half.
Ms McCall said the airline was benefiting from passenger loyalty. "There is an element here of repeat customers, so 58% of customers are rebooking, and 62% of business passengers are rebooking."
The airline said it was "well positioned to grow revenue and profit this year".
It predicted that the overall market for European short-haul flights would continue to grow, thanks to "improved economic conditions and a lower oil price".
However, it said April's strike disruption, plus the change in the timing of the Easter holiday period, meant third-quarter revenues - stripping out the impact of currency movements - would be lower.
"Perhaps the fact that the market has become accustomed to being pleasantly surprised has resulted in a sharp markdown to the shares, given that the French air traffic control strikes will impact on revenue per seat in Q3," said Richard Hunter, head of equities at Hargreaves Lansdown Stockbrokers.
"In addition, previously announced currency headwinds are also adding to the short-term gloom, whilst the general airline sector concerns such as geopolitical tensions are ever present in the background."