Eurocontrol, the European air traffic agency, said Britain and the Netherland were the worst affected areas, although it expected the situation to improve later on Monday.
"During the course of the day, the current cloud is expected to disperse," the agency said in a statement. "Delays will also be experienced by flights due to congestion in airspace adjacent to closed areas."
Britain's two biggest airports reopened after overnight closures, but passengers were warned to expect long delays and cancelations through the day.
Airports in Ireland and the Netherlands were also closed over fears that the drifting ash could damage jet engines and bring down aircraft.
The same Icelandic volcano's ash last month prompted a number of European countries to close their airspaces for nearly a week and travel chaos ensued in Europe and beyond.
British Transport Secretary Philip Hammond said aviation officials and aircraft makers were considering whether to allow planes to fly through higher densities of ash.
FLIGHT CANCELLATIONS
A spokesman for Europe's busiest airport Heathrow, in west London, said it had reopened at 0600 GMT, although delays and cancelations are expected. Gatwick, London's second biggest airport, will remain closed to flight arrivals until 1200 GMT. Nearly 150 arrivals and departures will be canceled on Monday morning, about half the scheduled total.
British flights face further disruption on Tuesday when British Airways cabin crew are due to strike in a long-running dispute over pay and working practices. Their union and BA managers were holding last-minute talks in London to avert the strike.
In Ireland, aviation officials said Dublin airport would reopen at 1100 GMT and the conditions looked likely to improve.
"The prospects for the next number of days are very good, we don't anticipate any problems with volcanic ash over the next two to three days," Irish Aviation Authority Chief Executive Eamonn Brennan told national broadcaster RTE's radio news.
In the Netherlands, Schiphol Airport in Amsterdam and Rotterdam Airport said there would be no flights from 0400 GMT until 1200 GMT or until further notice.
Officials advised passengers to stay away from airports and to contact their airlines instead.
KLM, part of Franco-Dutch airline Air France, said it diverted Amsterdam-bound flights already in the air to Paris, Dusseldorf, Maastricht and Frankfurt, with plans to transfer all of those passengers back to Amsterdam by bus. At least three US-bound flights were also canceled.
Air traffic control authorities in the Netherlands were not immediately available to say when a decision would be made about a reopening or an extension of the closure of Dutch airspace.
Amsterdam Schiphol is Europe's third-largest cargo airport and fifth-largest passenger hub.
More than 100,000 flights were canceled across Europe last month because of the volcanic ash forming a cloud over the continent. Millions of people were stranded and airlines, already battered by the global economic downturn, lost $1.7 billion, the International Air Transport Association has said.
The volcano under the Eyjafjallajokull glacier in Iceland has been erupting for weeks and the ash plume has reached heights of 25,000 feet (7,600 meters).
Volcanic ash contains tiny particles of glass and pulverised rock that can damage engines and airframes.
In 1982 a British Airways jumbo jet lost power in all its engines when it flew into an ash cloud over Indonesia, gliding toward the ground before it was able to restart its engines.
Channel Tunnel rail operator Eurostar said an extra 3,500 seats would be available for passengers.
