The volcanic ash cloud playing havoc with airline schedules will mean the loss of 1,000 flights in Europe on Monday with airports in Britain and the Netherlands hit, the Eurocontrol agency said.
The cloud was however expected to disperse over the day, Europe's intergovernmental air traffic coordinating agency said, as British and Dutch airports were closed because of the risks of flying through the ash.
"Today Eurocontrol expects 28,000 flights in Europe. This is approximately 1,000 less than on a normal day, and is due to the expected impact of the current closure of airspace in the south-east of the UK and in the Netherlands," it said.
"The areas of ash concentration are mainly at low levels. During the course of the day, the current cloud is expected to disperse somewhat," it said in one of its regular updates.
By 1200 GMT "the cloud is expected to mainly affect Northern Ireland, parts of Scotland and parts of south-west UK," it said.
There may also be some disruption to flights in the greater London area and there would be delays due to congestion in airspace adjacent to closed areas, Eurocontrol warned.
The ash cloud forced the closure of Amsterdam's Schiphol airport and those in Rotterdam and Groningen until 2:00 pm (1200 GMT) Monday, Dutch officials said.
London's main airports Heathrow and Gatwick reopened Monday after being forced to close overnight by the ash cloud billowing from the Eyjafjoell volcano in Iceland, but other British airports remained shut.
However there were no incoming flights at Gatwick airport in the morning and departing flights were subject to delay and cancellation.
Scotland's busiest airport, Edinburgh, plus Aberdeen and Inverness were closed while Wales's main airport Cardiff was shut, as was Swansea.
In England, Bristol in the southwest and Farnborough, southwest of London, were also closed until 1200 GMT.
The volcanic dust at more concentrated levels presents a danger to plane engines, though some industry officials have complained that the safety measures and airport closures have been excessive.
The latest ash closures came at the beginning of a week where air travel disruption was already expected due to a five-day strike by British Airways cabin crew set to kick off on Tuesday.
Much of European airspace was shut for up to a week in April following Eyjafjoell's eruption.
It was the biggest shutdown of the continent's airspace for more than 50 years.
Air travellers were advised to contact their airline for the latest flight information.
On Sunday, Eurocontrol said, disruptions in Ireland and the north-west of Britain resulted in a loss of some 400 flights.