Thousands of passengers face UK flight cancellations because of drifting ash from an Icelandic volcanic eruption.
Airports likely to be affected from 1300 to 1900 BST include Londonderry, Glasgow, Edinburgh, Prestwick, Durham Tees Valley, Newcastle and Carlisle, air traffic services company Nats said.
European air traffic controllers said 252 flights had been cancelled so far.
Ryanair said it had carried out a test flight over Scotland and challenged a ruling some flights should be grounded.
The carrier said its one-hour flight at 41,000ft showed there was no visible volcanic ash cloud or evidence of ash on the airframe, wings or engines.
It claimed the "red zone" over Scottish airspace where ash has been classified "high-density" was a misguided invention by the Met Office and the Civil Aviation Authority (CAA).
Meanwhile, the UK's emergency response committee, Cobra, will meet at 1530 BST to discuss the knock-on effects of the ash cloud.
Transport Secretary Philip Hammond, who will lead the talks, said although there would be widespread disruption for Scottish airports, it was unlikely to last long.
He told the BBC he expected Edinburgh and Glasgow airports to be clear by afternoon on Tuesday, and northern Scottish airports by Wednesday morning.
"At the moment the model suggests that disruption later in the week is likely to be limited, but of course the weather patterns are changing all the time," he said.
At 0800 BST the Met Office said the ash had already reached northern Scotland and would spread to Wales, northern England and Northern Ireland by midday and across much of the UK by the end of the day.
But forecasters said changing wind patterns make it hard to predict its exact path and concentrations would vary between regions.
The following airlines have announced cancellations:
British Airways is not operating any flights between London and Scotland until 1400 BST
KLM cancelled flights to and from Aberdeen, Glasgow, Edinburgh, and Newcastle as well as flights from Durham Tees Valley Airport
EasyJet cancelled flights from Glasgow until lunchtime
Ryanair said it has been advised by the Irish Aviation Authority not to operate flights from Glasgow Prestwick, Edinburgh or Aberdeen until at least 1300 BST on safety grounds, but believed that was no basis for the cancellations
Aer Lingus cancelled 12 flights to and from Glasgow, Aberdeen and Edinburgh
Flybe cancelled 11 flights to and from Scotland
BMI said flights to and from Aberdeen were subject to delay but services in and out of Glasgow and Edinburgh were unaffected
Glasgow-based Loganair has cancelled 38 flights. Only inter-island routes in Orkney are unaffected
Eastern Airways will not be operating any services in or out of Scottish airspace
Elsewhere, US President Barack Obama has been forced to shorten his visit to Ireland because of the ash.
And the situation is being closely followed by Barcelona FC, whose players are due to fly to London for Saturday's Champions League final against Manchester United at Wembley.
The cancellations come just over a year after another volcanic eruption in Iceland caused widespread disruption across Europe, including the closure of UK airspace, amid concerns about the damage volcanic ash could cause to engine aircraft.
This year, in the UK, the decision on whether to fly or not in ash cloud conditions is down to individual airlines, although they have to apply to the Civil Aviation Authority (CAA) for final approval.
The CAA said procedures were "totally different" compared with last year and although no airlines had applied to fly in high-density ash, some had applied for, and been given, permission to fly in medium ash.
Andrew Haines, from the authority, said: "It's a very fluid situation, partly because of the nature of the volcano, but also because the north of England and Scotland - as we've seen - have had very exceptional, windy conditions."
The Grimsvotn volcano in Vatnajokull National Park began erupting on Saturday and closed Iceland's airspace for a period.
Experts say the eruption is on a different scale to the one last year and ash particles are larger and, as a result, fall to the ground more quickly.
Frances Tuke, from travel industry body Abta, urged passengers to contact their airlines, which he said had legal obligations to their customers.
He said passengers could have a claim under European "denied boarding" regulations.
These state that if a flight is cancelled or delayed for more than five hours, passengers are entitled to be either re-routed, given a replacement flight, or a refund.
Since last year, the CAA has graded ash levels as low, medium or high, and airlines are notified if levels reach medium or high.
All British aircraft can fly in medium-density ash but the airlines need to consider whether to fly, according to risk assessments.
