Travel misery for thousands of passengers across Europe looks set to continue as the cloud of volcanic ash drifts across the continent.
Hundreds of flights have been cancelled due to the Icelandic cloud in the UK.
Today flights from Glasgow and Edinburgh International airports, as well as Dundee, were cancelled, with severe disruptions at other Scottish airports.
In England, Newcastle International was worst-hit.
A row also broke out between Irish airline Ryanair and the Civil Aviation Authority, after the airline's boss claimed there was "no volcanic ash" in Scottish airspace, and vowed to continue operating.
They sent a test flight into the "high ash concentration zone" and travelled from Glasgow Prestwick to Inverness, on to Aberdeen and down to Edinburgh.
An inspection of the aircraft afterwards found "no evidence of volcanic ash on the airframe, wings or engines", the company said.
"It's perfectly safe. There is nothing up there," he said.
He hit out at both the Civil Aviation Authority (CAA) - accusing them of incompetence.
"(They should) take their finger out of their incompetent bureaucratic backsides and allow the aircraft back into the skies over Scotland".
Mr O'Leary said the CAA's so-called "red zone" - an area of high-density volcanic ash - was "non-existent, mythical and a misguided invention".
He added: "The [Met Office] charts are rubbish. The CAA's decision to close Scotland is bureaucratic bungling."
However it was forced into an embarrassing U-turn just hours later, after the Irish Aviation Authority directed the airline cancel all Scottish flights.
A statement from the airline said: "Despite Glasgow Prestwick and Edinburgh Airports being outside the 'red zone' on the most recent UK Met Office charts, the UK Civil Authority have decided that these charts are wrong and have closed the airspace.
"Ryanair sincerely apologises for what we believe are unnecessary flight cancellations."
The CAA and Transport Secretary Philip Hammond rejected Mr O'Leary's claims.
"The CAA can confirm that at no time did a Ryanair flight enter the notified area of high contamination ash over Scotland this morning," a spokesperson said.
Mr Hammond added: "My information is that the Ryanair jet did not in fact fly into any of the 'red zones' designated by the CAA.
"We have the radar track of the aircraft [and] at the heights it was flying, and the areas it was flying and the times it was flying, the CAA model predicted - as Mr O'Leary discovered - that there would be no ash."
He went on: "We are confident of the modelling that is being used.
"It is open to the airlines to challenge that modelling - they can submit a safety case to the regulator suggesting that concentrations of ash are lower than the modelling shows.
"But they have to support that with some evidence and Ryanair has not produced any."
Mr Hammond will chair a meeting of the Government's Cobra emergency team later today as part of efforts to tackle the upheaval.
Meanwhile, British Airways said it was cancelling all flights to and from Glasgow, Edinburgh and Newcastle for the rest of the day.
Easyjet has suspended services going into and out of Glasgow, Inverness, Edinburgh, and Aberdeen and BMI has also cancelled all flights to and from Edinburgh and Glasgow.
As mentioned earlier, Ryanair has also now cancelled all scheduled flights to and from Scottish airports.
Elsewhere, various European airlines have seen their flights affected, including in Germany, where Hamburg, Bremen and Berlin may temporarily close.
Passengers have been urged to check with their airlines before travelling to airports.
Virgin Trains has provided around 3,000 extra seats on services running between London and Scotland to deal with additional demand.
Useful links for more travel information
:: British Airways
:: Brussels Airlines
:: Fly BMI
:: Eastern Airways
:: Gatwick Airport
:: BAA Airports (Heathrow, Stansted, Glasgow, Aberdeen, Southampton and Edinburgh)