British Airways carried 5.6% fewer passengers last month than in September 2007 and announced that trading conditions continued to be "challenging".
A total of nearly 2.8 million passengers flew with BA in September 2008, compared with more than 2.96 million in September 2007.
BA said forward bookings were being affected "by the increased anxiety in financial markets and by the uncertain economic outlook".
Africa and Middle East traffic dipped the most last month, dropping 10.1%. The UK and Europe sector fell 5.9%, North and South America including the Caribbean was down 2.6% and Asia Pacific decreased 5.3%.
BA planes were 74.0% full last month compared with 78.3% in September 2007.
Numbers travelling in first class and business class seats fell 8.6% while economy class traffic was down 4.1%.
BA said: " Revenue forecasts for the year carry some risk, although current good yields and the stronger dollar are broadly offsetting the volume impact.
"Cost initiatives continue in an effort to offset revenue risk. Fuel costs are still expected to be around £3 billion for the year. The target continues to be for the business to break even at the operating level."
Meanwhile, Irish budget airline Ryanair said it carried 5.23 million passengers last month - a 20% increase on the September 2007 total.
Its planes were 84% full in September, compared with 85% in the same month last year.