The return of the Icelandic volcanic ash cloud has caused travel misery for thousands of air passengers, with hundreds of flights cancelled.
Airlines had to scrap all services to and from Irish airports after no-flight restrictions were introduced until 1pm.
The ash cloud, borne on north-westerly winds, also prevented flights to and from Tiree, Barra, Benbecula and Islay in the Hebrides, as well as Campbeltown in Argyll in mainland Scotland.
Budget airline Ryanair cancelled all flights to and from Ireland, Belfast and Derry City from 6am to 2pm. Another major no-frills carrier, easyJet, had to axe around 20 flights on services to and from Belfast City and Belfast International airports.
Aer Lingus suspended all Irish services to the UK and Europe until 1pm. Heathrow said that around 20 flights to and from the west London airport had been cancelled.
"We are asking passengers to check with their airlines before coming to the airport," said a Heathrow spokeswoman.
Aer Arann was forced to cancel services to and from Derry and Donegal airports last night while easyJet warned of possible disruptions on services to Northern Ireland and Scotland.
British Tory leader David Cameron had to cancel a planned campaigning visit to Northern Ireland due to the flight restrictions, party officials said.
Flights from continental Europe were not affected by the ash cloud from Iceland's volcano Eyjafjallajokull, which caused travel chaos when it forced the closure of much of Europe's airspace for almost a week last month. Transatlantic services passing over Ireland and Northern Ireland were also unaffected.
Virgin Trains added extra carriages on its West Coast Main Line services from London that connect at Holyhead with ferries to and from Ireland.
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