British Airways (BA) has blamed a weak US dollar and changes in the timing of holidays for a dip in traffic figures during May from a year earlier.
BA is Europe's third-biggest airline
It was also hurt by carry-on luggage restrictions and said demand was weaker for non-premium tickets, especially on North Atlantic routes.
Earlier in the day, budget airline Ryanair also warned that higher taxes and airport charges would dent demand.
The airline industry as a whole is facing turbulent times, analysts say.
BA has been caught up in a number of problems, including being found to have broken competition law by colluding on fuel surcharges.
The airline was also found to be the most likely to lose a customer's luggage in Europe-wide study, and it had to cancel hundreds of flights just before Christmas as a result of bad weather.
On Tuesday, the company said that passenger capacity, measured in available seat kilometres, was 0.1% lower last month than in May 2006.
Traffic, when measured in revenue passenger kilometres, was 2.1% lower, pushing the passenger load factor down to 73.3%, 1.5 percentage points less than last year's level.
BA said that both its premium and non-premium traffic slid 2.1% during May.
The carrier said the timing of bank holidays had affected the traffic figures. It added that May 2006 was a "particularly strong month" but noted that "visibility around forward bookings continues to be limited".