Around three-quarters of flights scheduled in Europe Wednesday are expected to take place, the body coordinating air traffic control across the continent said.
"Eurocontrol expects approximately 21,000 flights to take place today in European airspace. On a normal Wednesday, we would expect 28,000," a statement said, updating on the impact of the volcano eruption in Iceland.
"Almost all of European airspace below 20,000 feet (6,096 metres) is available and air traffic services are being provided, although restrictions are still in force in some areas," it said.
Sweden and Helsinki were under restrictions, but Eurocontrol said it expected those measures to be lifted during Wednesday. All European airspace above 20,000 feet was open at 0900 GMT, it said.
The organisation said that by the end of Wednesday it expected that a total of more than 100,000 flights would have been cancelled since April 15 when the cloud of volcanic ash first spread across northern Europe.