The ash cloud from the erupting Iceland volcano could force the cancellation of up to 500 commercial flights in Northern Ireland, Scotland and parts of Scandinavia, the European air traffic agency has said.
Between 200 and 250 flights have already been cancelled across Europe, with more expected, said Brian Flynn, head of network operations at Eurocontrol.
The announcement comes after the British government promised disruptions would be minimised and there would not be any shutdowns.
"We've moved on to a different way of working, we won't be closing airspace," Philip Hammond said this morning.
The Transport Secretary, will chair a meeting of the Government's emergency planning committee, Cobra, today to discuss the latest situation.
Thousands are expected to be affected by the disruption with over 30,000 flights in Europe, on average, every day at this time of the year.
Ryanair, never keen to slink away from a confrontation, said it strongly objected to the disruption under orders from the Irish Aviation Authority (IAA).
The IAA said last night that it did not expect any disruption at Irish airports during most of today.
But it later ordered flights to Scotland to be cancelled as forecasters predicted that the volcanic ash cloud from the volcano would hit Scotland and Northern Ireland during the morning, with much of the UK being covered by midday.
Ryanair insisted there was no basis for the flight cancellations.
The airline completed a one-hour "verification flight" up to 41,000ft in Scottish airspace this morning. The aircraft took off from Glasgow Prestwick, flew to Inverness, on to Aberdeen and down to Edinburgh.
In a statement, a spokeswoman for the Irish airline said: "There was no visible volcanic ash cloud or any other presence of volcanic ash and the post-flight inspection revealed no evidence of volcanic ash on the airframe, wings or engines.
"The absence of any volcanic ash in the atmosphere supports Ryanair's stated view that there is no safety threat to aircraft in this mythical 'red zone', which is another misguided invention by the UK Met Office and the CAA (Civil Aviation Authority)."
Weather forecasters have predicted there will be some respite over the next few days with the wind shifting direction, blowing the ash away from British airspace.
Glasgow Airport is operating a limited schedule until 7pm after the Civil Aviation Authority confirmed a high concentration of ash in its airport. This morning the airport was able to operate 40 per cent of its flights.
There have been cancellations at Edinburgh, Inverness, Aberdeen Newcastle and Durham Tees Valley airports.
Despite the severity of the Grimsvötn eruption, Iceland's biggest in 50 years, the disruption to aviation has not been on the same scale as last year, when the ash cloud from the Eyjafjallajökull volcano crippled aviation throughout Europe.
They say procedures have been improved since then and the cloud is currently not expected to move over continental Europe.
The volatile weather has kept the volcanic ash moving, according to the Met Office, who said that its latest forecasts showed the skies clearing tomorrow.
Some ash is expected to move into British airspace on Thursday before clearing again on Friday.