Northern Ireland airspace has closed again after a brief window of opportunity for travellers on Tuesday morning.
The air traffic control body, Nats, said airspace would be closed from 1300 BST to 1900 BST as a new volcanic ash cloud spreads towards the UK.
Earlier, some flights left Belfast City Airport, bound for Scotland.
At the International Airport, flights have been cancelled, except those to the Isle of Man.
The volcano eruption in Iceland has strengthened and the new cloud is spreading south and east towards the UK. The eruption had abated for a time on Monday morning.
Nats, said the situation was "dynamic and rapidly changing".
Prime Minister Gordon Brown said airports were "taking advantage of the window of opportunity" as the impact of the volcano ash cloud temporarily lessened, but stressed that passenger safety would remain "paramount".
Stranded
He added: "In the meantime I think it's important that everybody knows that if they can get to a Channel port we can get them across from Europe to the United Kingdom".
The airline said it would operate additional flights on Wednesday and Thursday from Fuerteventura, Gran Canaria, Lanzarote and Tenerife to Madrid.
This would allow passengers to get to mainland Europe and make their return journeys via road, rail or ferry. Onward travel from Madrid will be at each passenger's own expense.
This is the sixth day of flight cancellations across the UK.
It is estimated 120,000 passengers have been affected by the closure of Northern Ireland airspace.
Thousands of passengers remain stranded with planes grounded across Europe.
Flybe did operate a limited service to some Scottish airports from Belfast City. But flights for the rest of the day are cancelled.
EasyJet said flights to and from Northern Europe, including UK flights, would be cancelled until 1800 BST on Tuesday.
Aer Lingus said all flights scheduled for Tuesday are cancelled.
Ryanair has cancelled all scheduled flights to and from the UK and Ireland until 1300 BST on Wednesday.
BMI plans to resume some flights from Heathrow at 1900 BST, but all flights from UK domestic airports are cancelled until at least 2359 BST.
Jet2 and Thomson flights will continue to be cancelled until Thursday at least. Passengers are advised to check airline websites for updates.
Aer Arann said its flights from Dublin to City of Derry Airport are cancelled.
The Irish Aviation Authority (IAA) has announced that Dublin Airport will remain closed for all inbound and outbound commercial flights until 1900 BST.
Shannon Airport is expected to reopen from 1300 BST but the authorities said no flight would depart this evening.
Cork Airport remains closed and a further update will be provided later.
More than 6.8m passengers have been affected so far and 63,000 flights have been cancelled since Thursday.
Experts say the tiny particles of rock, glass and sand contained in the ash cloud from the Eyjafjallajoekull volcano system could jam aircraft engines, as has happened in previous incidents of planes flying into plumes of volcanic ash.
BBC News
