Airports in Ireland and parts of Britain were closed again for hours on Tuesday because of the cloud of volcanic ash drifting south from Iceland that wreaked havoc on European air travel last month.
The Irish Aviation Authority (IAA) allowed flights to resume from Irish airports from 1200 GMT after a closure lasting six hours. Dublin airport said flight schedules had reverted to normal operations.
Britain's National Air Traffic Services (NATS), which had imposed a no-fly zone in the west of Scotland and Northern Ireland, also opened all UK airspace from 1200 GMT, apart from a small no-fly zone in the northwest with no impact on UK flights.
Flights in much of continental Europe were operating as normal.
However, the IAA said northerly winds forecast for the coming days could bring more clouds of ash from the Icelandic eruption and disruption for passengers this week.
"We could be faced with this periodically during the summer," IAA Chief Executive Eamonn Brennan said. "We are probably facing a summer of uncertainty due to this ash cloud."
The IAA had closed airports from 0600 GMT due to risk of ash ingestion in aircraft engines, although overflights of Ireland from Britain and continental Europe had not been banned.
European flights overall were expected to be at almost normal levels on Tuesday, European air traffic agency Eurocontrol said.
On a typical weekday, just under 200 flights would be expected in and out of Ireland, compared with around 28,000 throughout Europe, a spokeswoman said.
However, fresh ash from the erupting volcano could cloud airspace over northern Britain for the next few days following a change in direction of the wind, Iceland's meteorological office said.
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.
The airspace closures cost Europe's airlines 1.5 billion to 2.5 billion euros ($2 billion-$3.3 billion), the European Commission has estimated.
Former Irish state airline Aer Lingus said that last month's shutdown of several days cut its earnings by about 20 million euros ($27 million). The final bill will hinge on the impact on passengers' longer-term travel plans.
Shares in Aer Lingus, which also separately reported encouraging first-quarter trading on Tuesday, dropped 4.11 percent by 1232 GMT, while Ryanair, which is much less dependent on the Irish market, dropped 1.61 percent.
Airlines Air France-KLM, British Airways, Iberia and Lufthansa were between 1.58 and 4.12 percent weaker, in a lower overall market.
"While the current airspace restrictions are likely to be brief, we can't rule out persistent sporadic air travel disruptions in parts of Europe going forward depending on ash cloud movement," Deutsche Bank commodities analyst Michael Lewis said.
Additional reporting by Marie-Louise Gumuchian in Dublin, Michael Holden and Caroline Copley in London, Philip Blenkinsop in Brussels and Maria Sheahan in Frankfurt; editing by David Stamp and Mark Heinrich.