Low-cost carrier easyJet said passenger numbers increased 15.9 per cent in May compared to last year after its acquisition of GB Airways.
The budget airline sold a total of 3.88 million seats over the month, an extra 532,495 from last year.
But its load factor, which represents the number of unfilled seats on flights, fell from 83.6 per cent in 2007 to 83.2 per cent in 2008.
The drop represents the second month in a row of decline for easyJet.
Low-cost airlines need to fill most of their seats on every flight in order to make a profit, and shares fell by 4.65 per cent in morning trading.
The airline has previously warned margins will be squeezed by dramatic increases in oil prices and that the additional costs will damage profits this year.
Another oil-price surge yesterday, linked to a fall in the value of the dollar, led to a sell-off in airline shares this morning.
British Airways, which reported a drop in its passenger traffic on Wednesday, saw the value of its stock fall by 4.52 per cent by mid morning.
In the 12 months through May, easyJet recorded a 15 per cent rise in passenger numbers to 40.6 million, while the load factor fell 0.8 percentage points to 83.2 per cent.