Two airports serving Glasgow, Scotland's largest city, were closed until at least 1800 GMT, while flights from the Irish capital Dublin face restrictions until at least 2300 GMT.
The latest disruption signaled that travel hold-ups would continue into the summer holiday period because of ash being blown from the same volcano in Iceland that caused mayhem for 10 million travellers last month.
The European air traffic agency Eurocontrol said about 300 of 29,000 scheduled flights were likely to be cancelled 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 northwest 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 cancelled assuming a general strike in Greece against austerity plans lasted until midnight.
Britain's official weather forecaster, the Met Office, said predictions indicated the ash would remain over much of Ireland, Scotland and western England into Thursday and would continue to move south.
British Airways said it was cancelling all flights to and from the Scottish cities of Glasgow and Edinburgh until 1800 GMT.
Irish budget airline Ryanair also cancelled 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, based on current forecasts, it did not expect problems in the southeast, where the major airports serving London are located.
"The situation remains changeable, so passengers expecting to travel today and tomorrow from airports in Scotland, Northern Ireland, the North West of England, Wales and the West Country should contact their airlines to check whether their flight is operating," the CAA said in a statement.
Much of European air traffic was grounded last month because of the spread of ash from the erupting volcano under the Eyjafjallajokull glacier in Iceland.
Some 100,000 flights were cancelled 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 criticised 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.
