Some weekend European flights were grounded by the ash cloud
Airports across Europe have reopened after volcanic ash from Iceland disrupted flights at the weekend.
European air safety body Eurocontrol said the cloud of ash had dispersed. Lisbon airport in Portugal reopened on Monday at 0900 (0800 GMT).
But flights were still being rerouted around ash clouds over the Atlantic, causing delays, Eurocontrol said.
Meanwhile, Ryanair grounded two of its planes at Belfast City Airport after ash traces were found in their engines.
A spokesman for the budget airline said there was "no risk" to the planes and that they would return to service later on Monday.
Eurocontrol said no airports were closed in Europe, but that it was expecting some 28,500 flights within the European area, about 500 below the average for this time of year.
Abta's Frances Tuke says volcanic ash is hitting air travel less than in April
However, Portugal's civil aviation authority said airports on the Azores and Madeira islands remained shut after a weekend of closures across parts of the continent.
Airlines and airports are continuing to urge passengers to check details of their flights before travelling to the airport, with warnings of periodic disruption throughout the summer.
Last month, ash from the Eyjafjallajokull volcano shut down airspace across Europe for five days.
And it looks set to be a continuing cause of frustration for the travel industry, with recent images showing activity in the volcano increasing and emitting ash up to 20,000ft (6,000m).
Airport operator BAA said the disruption cut the number of passengers using its British airports by more than a fifth last month - compared to April last year.
Wolfgang Prock-Schauer, chief executive of airline BMI, said EU rules on predicting ash concentrations were "not accurate enough" and led to "unnecessary closure of airspace".
He urged regulators to work more closely with the airport industry, adding that the theoretical model for assessing risk had not been tested properly.
"The experience of other areas of the world with high volcanic activity has to be taken into account to bring European air traffic back to normal," he said.
More than 300 flights were grounded over the weekend in Portugal, AFP news agency reported.
France and Austria grounded some flights on Sunday night.
French authorities grounded at least 70 flights bound for southern Europe from airports in Paris, Lyon, and Nice.
Spanish airspace began returning to normal on Sunday after 19 airports in the north of the country were closed on Saturday.
In Italy, airports at Milan, Pisa and Florence reopened after midday on Sunday, but disruption was widespread and there were delays for UK-bound flights.
In Scotland, airports in Inverness, Kirkwall, Wick, Benbecula and Stornoway were shut for a time, but UK air traffic control service Nats later said there were no airspace closures.
And the Irish Aviation Authority imposed restrictions on Donegal, Sligo and Knock airports in the west of the country on Sunday afternoon.