Flights were grounded today after ash levels in the atmosphere once again made it unsafe to fly.
And tonight there were concerns the Icelandic ash cloud could disrupt more flights tomorrow.
Borne on winds from the north, the ash cloud caused the closure of airspace in the Republic of Ireland and in Northern Ireland until 1pm today.
Hundreds of flights had to be cancelled, with services in the Hebrides and parts of northern mainland Scotland also affected.
Last month the ash cloud forced the closure of much of Europe's airspace for almost a week.
Tonight the UK's Civil Aviation Authority was due to announce whether Scottish airspace would be affected tomorrow.
Budget airline Ryanair cancelled all its flights to and from the Republic of Ireland, Belfast and Derry City from 6am to 2pm today.
The Irish no-frills carrier put on extra flights this afternoon and plans more for tomorrow to help those caught up in the latest disruption.
Earlier, easyJet axed around 20 flights on services to and from Belfast City and Belfast International airports, while Irish airline Aer Lingus suspended all Irish services to the UK and Europe until 1pm.
Around 20 flights were cancelled at Heathrow and the ash cloud also prevented morning services to and from Tiree, Barra, Benbecula and Islay in the Hebrides, and Campbeltown in Argyll in mainland Scotland.
Flights from continental Europe were not affected by the ash cloud from volcano Eyjafjallajokull.
Transatlantic services passing over Ireland and Northern Ireland were also unaffected.
As travellers struggled to cope with the disruption, the Association of British Insurers said insurance companies were likely to have to pay out around £62 million to those affected by last month's ash-cloud chaos.
Among those plunged into fresh travel gloom today were US friends Renee Meadors and Rachelle Velgersdyk.
Due to board an early-morning flight to Prague, they were instead preparing to bed down in Dublin airport.
The 23-year-olds had already spent a week stranded in Spain after last month's airports' shutdown.
"I can't believe it's happened again today of all days," Miss Meadors said.
"The soonest we could get another flight to Prague is next week so we're going to fly to Birmingham tomorrow morning and go from there."
To add to travellers' problems, a 24-hour general strike in Greece has led to airlines axing their Greek services tomorrow.Reuse content