Aviation chiefs grounded flights into and out of Ireland and Northern Ireland on Tuesday as ash from an Icelandic volcano drifted in, causing fresh travel chaos for thousands of passengers.
After last month's lengthy airspace shutdown across Europe that crippled air travel, Irish and British air authorities announced a new aerial closure from 0600 GMT due to the risk to plane engines from the ash.
"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 -- would be closed from 0600 GMT.
"Airspace over Northern Ireland will also be closed from 0700 local time (Tuesday) morning," it said in a statement.
The air authority also closed down airspace over the Outer Hebrides islands off the northwest of Scotland at 1700 GMT Monday due to the ash cloud.
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 from 0600 GMT to 1200 GMT Tuesday due to the dangers posed by the new volcano cloud.
"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."
Information from the London-based Volcanic Ash Advice Centre (VAAC) suggested that the ‘no fly-zone' would affect Dublin and other airports across the country, said the IAA.
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 Tuesday.
"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.
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