Heathrow airport saw 320,000 passengers travel in May, a 6% drop in expected traffic.
An overall 5% drop across its UK airports was a result of volcanic ash and strike action at British Airways, said BAA.
Glasgow and Edinburgh airports also saw a 6% drop. Aberdeen and Southampton performed better, with a 4% and 3% drop in expected traffic respectively.
Stansted Airport had the strongest performance, but still saw a 2% fall in expected traffic.
www.baa.com