Air traffic control authority Nats has said the number of flights it handled in April dropped 21.2% from last year due to the volcanic ash crisis.
Flights over British airspace were down from 180,166 in April 2009 to 141,956 last month, the company said.
It added that figures for May are also likely to dip since the ash problem continues to force flight restrictions, which means air passengers should make sure they have adequate travel insurance before heading overseas.
Ian Hall, Nats' director of operations, strategy and standards, said: "For the UK's aviation industry, the widespread presence of volcanic ash was an unprecedented event. We worked with the regulator, government and our customers to ensure safety was the first priority.
"We are focused on delivering the safe and efficient regeneration of air traffic to support the airlines in recovering their operations. This natural hazard, the scale of which has not been experienced in this region before, continues to threaten air travel and we continue to monitor volcanic activity data to try to minimise the effect on our airspace."
Airline services have largely managed to return to normal thanks to changing winds and a new set of measures to allow more flights to operate in ash cloud conditions.
© Press Association 2010