"August produced another negative figure in the relentless series of traffic decreases which stretches back to November 2008," AEA said in a statement.

"The latest result was minus 1.8 percent in passenger kilometres and minus 3.5 percent in passenger boardings," it added. "The preliminary outlook for September ... is for a figure worse than in August, at about minus 3 percent (in passenger-kilometres)."

Passenger-kilometres is a measure that takes account of the distance travelled by customers as well as their numbers.

(Reporting by Pete Harrison; Editing by David Holmes)