International airline traffic fell by 2.4% in April as cancellations caused by the volcanic ash cloud from Iceland led to fewer flights.
According to the International Air Transport Association (Iata), European carriers were hit the hardest, with traffic dropping 11.7%.
And North American carriers reported a 1.9% fall in April as north Atlantic routes were also hit.
Iata's data excludes domestic flights, and represents more than 240 airlines.
"The ash crisis knocked back the global recovery - impacting carriers in all regions," said Giovanni Bisignani, director general of Iata.
'Salt in wound'
Referring to March's figures, he said that they had been "within 1% of pre-crisis traffic levels in 2008".
"In April, that was pushed back to 7%."
He said the ash cloud crisis "could not have come at a worse time" for Europe.
"Europe's slow recovery from the global financial crisis and its currency crisis are already a huge burden on the profitability of its airlines."
And he said uncoordinated and excessive cancellations, and unfairly onerous passenger care requirements, had "rubbed salt into the European industry's wounds".