Ryanair called for restrictions on airlines to be lifted after it claimed to have flown a plane safely though the thickest part of the volcanic ash cloud.
The airline said it strongly objected to the disruption under orders from the Irish Aviation Authority which saw flights cancelled at Glasgow, Edinburgh and Aberdeen.
Ryanair said it had 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 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).
''Ryanair has also received written confirmation from both its airframe and engine manufacturers that it is safe to operate in these so-called 'red zones' and, in any event, Ryanair's verification flight this morning also confirms that the 'red zone' over Scotland is non-existent.''
Flights were cancelled after forecasters predicted that the volcanic plume, billowing from Iceland's Grimsvotn volcano, would hit Scotland and Northern Ireland during the morning, with much of the UK being covered by midday.