Ryanair has cancelled its noon flight from Dublin to Malta as a cloud of ash from a volcano in Iceland returned over Ireland today.
Aviation chiefs grounded flights into and out of Ireland and Northern Ireland as the cloud drifted south. Some airspace over Scotland was also affected but services to England were operating normally.
"Ireland falls within the predicted area of ash concentrations that exceed acceptable engine manufacturer tolerance levels," said the Irish Aviation Authority (IAA).
Irish flag carrier Aer Lingus and budget carrier Ryanair cancelled hundreds of flights, throwing travel plans for air passengers into disarray.
Britain's Civil Aviation Authority (CAA) said airspace over Northern Ireland -- a British province -- was also closed.
The air authority also closed down airspace over the Outer Hebrides islands off the northwest of Scotland late yesterday.
The new alerts should not disrupt aircraft overflying Ireland from Britain or Europe, or southern British airports including Heathrow, Europe's busiest air hub, authorities in the two countries said.
The cloud of ash came from the eruption of Iceland's Eyjafjjoell volcano, whose drifting dust was behind last month's shutdown that left hundreds of thousands of travellers stranded across the globe.
Airspace was re-opened after about a week following emergency talks between European governments, airlines and regulators.
The IAA said all flights into and out of Ireland would be grounded at least till 1200.
"The decision is based on the safety risks to crews and passengers as a result of the drift south of the volcanic ash cloud caused by the north easterly winds," said the authority in a statement.
It added that "over-flights of Ireland from the UK and Europe will not be impacted tomorrow. Flights in mainland Europe will operate normally."
Hundreds of flights were due to depart and fly into Dublin airport throughout the day, with more from Shannon and Cork in the south of the country and Ireland's smaller regional airports.
Ryanair said it had cancelled all flights into and out of Ireland between 0500 GMT and 1300 GMT.
"The first wave is clearly one of the busiest parts of the day so it will have a fairly significant effect on the operation tomorrow," airline spokesman Stephen McNamara told the BBC.
Irish flag carrier Aer Lingus said it had cancelled all British and European flights scheduled to depart and arrive into Dublin, Cork, Shannon and Belfast airports before 1200 GMT Tuesday.
IAA chief executive Eamon Brennan struck an upbeat note, however, saying it was hoped restrictions could be removed later Tuesday.
"We are quite optimistic that it will dissipate and we are quite optimistic for Dublin and for Shannon (airports) tomorrow afternoon but we will make a reassessment for that in the morning," he told the BBC.
The international airline industry body, IATA, said last month's shutdown cost carriers some 1.7 billion dollars (1.3 billion euros) and called on governments to pick up at least part of the cost, angered by their handling of the crisis.
Eurocontrol, the continent's air traffic control coordinator, said more than 100,000 flights to, from and within Europe had been cancelled between April 15 and 21, preventing an estimated 10 million passengers from travelling.
TRANSPORT MALTA STATEMENT
Transport Malta said it wished to remind passengers of affected flights that even in these extraordinary circumstances, airlines are still bound by their obligations under Regulation 261/2004 which stipulates that stranded passengers are entitled to be provided with duty of care by the airlines including the provision of accommodation and meals, during the time they are stranded.
Although the Irish Civil Aviation Authorities have cleared Irish Airports to open for full operations from 11 p.m. today, Transport Malta will keep monitoring the situation as things may change. Transport Malta will be issuing updates as necessary to the media to keep the public informed about developments.