EasyJet shares slumped today as the budget carrier said a strike by French air traffic controllers last month would hit profits.
The warning came after the British airline said it had managed an unlikely profit during its first half, a period when carriers normally report losses, thanks to lower fuel costs and a stronger pound.
But EasyJet shares closed almost 10% lower in London after it said that April's strike action by French air traffic controllers would hit pre-tax profits by £25m.
Analysts said that cheaper fuel costs helped the airline's first half results as expected, but French strike action as EasyJet embarks upon its key summer travel season has ruffled investors.
The British airline reported a net profit of £5m in the six months to the end of March compared with a loss after tax of £41m a year ago.
EasyJet, like its rivals, tends to post losses during the northern hemisphere winter when it carries fewer passengers compared with the peak summer holiday season.
"As expected the lower oil price has been beneficial for customers and EasyJet will be able to offer its customers even better value fares this summer whilst continuing to grow revenue and profit," the airline said in an earnings statement.
Oil prices slumped for most of the reporting period on a supply glut, making the cost of jet fuel - which is refined from crude - cheaper.
EasyJet had said in late March that earnings had benefited during its first half from currency movements after the European single currency slumped to multi-year low points against the pound and dollar on the back of euro zone strains.