The ash cloud crisis led to air traffic control company Nats handling 21.2% fewer flights last month compared with April 2009, it was announced today.
There were 141,956 flights in UK controlled airspace in April 2010, compared with 180,166 in April last year, Nats said.
At one stage in mid-April the ash levels from the Icelandic volcano eruptions were severe enough to prohibit all flights to and from UK airports.
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."
The May 2010 figures from Nats will also be depressed due to recent flight restrictions imposed by the continuing ash problem.
Changing winds and a new set of measures to allow more flights to operate in ash cloud conditions mean airline services have returned to normal.