Irish budget carrier Ryanair has described as unnecessary restrictions on flights to and from Scottish airports as a result of the latest volcanic ash cloud.
The airline said it had been told by the Irish Aviation Authority (IAA) that it could not operate 36 flights to or from Scottish airports on 24 May and that as a result these had been cancelled.
It said it had conducted a one-hour verification flight over Scotland on the morning of 24 May at an altitude of 41,000ft (12,500m). Scottish airspace was predicted to be affected by high-density concentrations of volcanic ash from Iceland's erupting Grimsvotn volcano.
The flight's route took it from Glasgow Prestwick over Inverness, Aberdeen and Edinburgh, all areas cited by UK Meteorological Office predictions as being in the 'red zone' of high-density concentrations of ash.
"There was no visible 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," said the airline in a statement.
"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'." It described the zone as "a misguided invention by the UK Met Office and the CAA".
The airline added that it has "written confirmation from both its airframe and engine manufacturers that it is safe to operate in these so-called red zones". The carrier operates a fleet of Boeing 737-800s powered by CFM International CFM56-7Bs.
Ryanair was vocal last year in its complaints that wholesale closure of large areas of European airspace during the eruption of the Eyjafjallajokull volcano was unnecessary.
Meanwhile, the IAA said that Irish airspace was unaffected by the volcanic plume. While it was difficult to look far ahead, the prognosis for avoiding airspace problems over Ireland was "quite optimistic".
Weather in the northeast Atlantic is said to be very changeable, which is making modelling of the plume's progress difficult.
Irish regional carrier Aer Arann has cancelled two flights between Dublin and Derry; the latter airport is on the edge of the predicted high concentrations of ash.