Between 200 and 250 flights had already been canceled, said Brian Flynn, head of network operations at Eurocontrol. There are just over 30,000 flights within Europe on average every day at this time of the year.
Ash from the Grimsvotn volcano already forced President Barack Obama to shorten a visit to Ireland, and has raised fears of a repeat of huge travel disruptions in Europe last year when ash from the Eyjafjalljokull volcano stranded millions of passengers.
Flynn said he did not expect Obama's European tour to be further affected and authorities have said they don't expect the kind of massive grounding of flights that followed last year's eruption.
They say procedures have been improved since then and the cloud is currently not expected to move over continental Europe.
Nonetheless, the British Civil Aviation Authority said strong winds over Scotland was making it hard to predict the direction in which the ash would move.
British Airways suspended all its flights for Tuesday morning between London and
Scotland, while Dutch carrier KLM and Easyjet canceled flights to and from Scotland and northern England at the same time.
Three domestic airlines also announced flight disruptions. The Irish Aviation Authority meanwhile ordered all flights from Ireland to Scotland to be suspended.
Budget airline Ryanair said it did not believe there was a need to cancel all flights between Ireland and Scotland, and said it would meet with regulators Tuesday morning to discuss the issue.
Norwegian airport operator Avinor said the ash disrupted traffic to and from Stavanger and Karmoey airports and is expected to spread to southern Norway later.
Danish authorities say they have closed the airspace under 6 kilometers (4 miles) in northwestern Denmark. There is no airport in that area, but the ash is causing delays and some cancellations at Copenhagen airport.
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