The decision was taken by the Civil Aviation Authority (CAA) following an updated forecast from the Met Office, with the quantity of ash now exceeding that considered safe by aircraft engine manufacturers.
Glasgow's two airports are expected to remain closed until 18:00, while restrictions at Dublin Airport began at 10:00.
British Airways has said it has cancelled all flights to and from Glasgow, Edinburgh and Aberdeen from midday, while cheap flights carrier Ryanair will not operate services to Scotland and Northern Ireland until at least this afternoon.
Flights to Scotland and Northern Ireland have also been suspended from East Midlands Airport, with passengers due to travel to either country advised to contact their airline before travelling.
The CAA has said the situation is changeable amid fears that the ash could begin disrupting people's summer holiday travel plans.
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