Shares in British Airways fell 8% after the airline said its costs this financial year would increase sharply.
The airline told an investor briefing that its total costs could rise by £300m-400m.
Sources familiar with the briefing said the airline attributed the rise to higher pension, fuel and wage costs.
BA was the second worst performer in London's FTSE 100 index of leading shares on Thursday, dropping 24.5 pence to 284.5p.
BA's financial chief John Rishton also warned investors that average fare prices would continue to come under pressure - and may even fall.
When the firm unveiled its results last month it said demand for short-haul flights remained weak in the face of tough competition from budget airlines.