British aviation chiefs also closed down airspace over a small area of northwest Scotland late Monday and said the shutdown would be extended to Northern Ireland the following day.

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.

Airspace across Europe was closed down for up to a week last month after the eruption of Iceland's Eyjafjallajokull, but was re-opened after emergency talks between European governments, airlines and regulators.

The Irish Aviation Authority (IAA) said Monday all flights into and out of Ireland would be grounded from 0600 GMT to midday the following day due to the dangers posed by the new volcano cloud.

"Ireland falls within the predicted area of ash concentrations that exceed acceptable engine manufacturer tolerance levels," said the authority in a statement.

"The decision is based on the safety risks to crews and passengers as a result of the drift south of the volcanic ash cloud."

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.

Budget Irish carrier 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," said airline spokesman Stephen McNamara.

Irish flag carrier Aer Lingus said it had cancelled all British and European flights scheduled to depart and arrive into Dublin and other airports in the country until 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.

Meanwhile Britain's Civil Aviation Authority (CAA) closed down airspace over the Outer Hebrides, an island chain off the northwest of Scotland, at 1700 GMT Monday due to the danger from the ash.

"Increased concentrations of volcanic ash in the atmosphere are causing airspace closures in isolated parts of Scotland," said the CAA in a statement.

The air safety watchdog also ordered a closure of Northern Irish airspace between 0600 GMT and 1200 GMT -- in line with the closure in the Republic of Ireland -- which included airports in Belfast.

"Airspace over Northern Ireland will also be closed from 0700 local time (Tuesday) morning," said the CAA.

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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