Two airports serving Glasgow, Scotland's largest city, were closed until 1800 GMT, while flights from the Irish capital Dublin faced restrictions from 1000 GMT until at least 2300 GMT.
The latest disruption, which could signal that travel glitches will continue into the summer holiday period, was caused by ash being blown from the same volcano in Iceland that caused mayhem for 10 million travelers last month.
The European air traffic agency Eurocontrol said about 300 of 29,000 scheduled flights were likely to be canceled across Europe on Wednesday. "The situation is not expected to improve in this area during the day," the agency said in a statement.
"The whole of Ireland, west Scotland and north-west England could be affected, with risk to operations at Manchester and Liverpool airports," it continued.
In addition, according to Eurocontrol, roughly 900 flights in Greek airspace would be canceled assuming a general strike in Greece against austerity plans lasted until midnight.
A graphic on the website of Britain's official weather forecaster, the Met Office, suggested the ash cloud would remain over much of Ireland, Scotland and western England into Thursday.
British Airways said it was canceling all flights to and from the Scottish cities of Glasgow, Edinburgh and Aberdeen until 1200 GMT.
Irish budget airline Ryanair also canceled flights to and from Glasgow Prestwick and the Northern Irish cities of Belfast and Derry until 2300 GMT.
It also warned that services at the English airports of Liverpool, Bristol, Leeds and Manchester could be affected.
Britain's Civil Aviation Authority said it did not anticipate problems in the southeast, where the major airports serving London are located.
"The situation now is what I would describe as more fluid and for that reason it's much more difficult to predict which airports will be open," Ian Hall, a director at Britain's National Air Traffic Service, told BBC TV.
Passengers were warned to check with their airlines before going to airports that were facing possible problems.
Much of European air traffic was grounded last month because of the spread of ash from an erupting volcano under the Eyjafjallajokull glacier in Iceland. Some 100,000 flights were canceled and millions of passengers stranded.
Tuesday was the first test of a European system of progressive closures, including partial no-fly zones, introduced after the ash cloud prompted a blanket ban that was criticized by airlines forced to ground thousands of flights in April.
European transport ministers have agreed to set safety limits for flying through the ash, which can paralyze jet engines, and to unify European airspace.
Last month's airspace closures cost Europe's airlines 1.5 billion to 2.5 billion euros ($2 billion-$3.3 billion), the European Commission has estimated.
