Flights in and out of Scotland have been cancelled as a volcanic ash cloud from Iceland heads towards the UK.
BA, KLM, Easyjet, Flybe, Aer Lingus, Loganair and Eastern Airways have cancelled services on Tuesday, and some flights over the Atlantic were delayed.
The threat of further disruption led US President Barack Obama to fly out of the Republic of Ireland a day early to get to London for a state visit.
Ash from another Icelandic volcano led to huge disruption in Europe last year.
Air Force One
Mr Obama had been due to fly to the UK on Tuesday morning, but White House deputy press secretary Josh Earnest said the decision to bring his arrival forward had been taken "due to a recent change in the trajectory in the plume of volcanic ash".
The Met Office forecasts the ash cloud will reach northern and western Scotland overnight, and will clip northern parts of Northern Ireland early on Tuesday. None of England is likely to be affected.
A Met Office spokesman said it was difficult to forecast the cloud's direction beyond that because weather systems were changing so rapidly.
A number of airlines are choosing not to fly through Scottish airspace on Tuesday:
- British Airways is not operating any flights between London and Scotland until 1400 BST
- KLM cancelled flights to and from Aberdeen, Glasgow and Edinburgh, as well as some to Newcastle
- EasyJet cancelled flights to and from Edinburgh, Glasgow, Inverness and Aberdeen scheduled for between 0500 and 0900 BST
- Flybe cancelled flights to and from Aberdeen and Inverness
- Aer Lingus cancelled a number of its flights between the Republic of Ireland and Edinburgh, Glasgow and Aberdeen
- Glasgow-based Loganair has cancelled 36 flights. Only inter-island routes in Orkney are unaffected
- Eastern Airways will not be operating any services in or out of Scottish airspace
Transport Secretary Philip Hammond told BBC Two's Newsnight that "most, if not all, flights into and out of Glasgow and Edinburgh and airports to the north will be stopped" on Tuesday morning.
But he said services should resume from Glasgow and Edinburgh by about lunchtime, and in other airports by Wednesday morning. Any disruption later in the week should be "limited", he added.
Earlier, Mr Hammond said there had already been "modest delays" to flights, particularly those crossing the Atlantic.
"Clearly, this is a natural phenomenon which we cannot control, but the UK is now much better prepared to deal with an ash eruption than last year."
A spokesman for Edinburgh Airport said it was anticipating disruption to many services on Tuesday.
In a statement on Monday evening, he said: "Only Ryanair is intending to operate a full service from Edinburgh Airport. Passengers should not travel to the airport without checking with their airline first regarding the status of their flight."
Andrew Haines, chief executive of the Civil Aviation Authority, said he hoped to avoid a repeat of last year's travel chaos, but he admitted it was still unclear how badly flights would be affected.
"We know so much more about the volcanoes. We have an improved model.
"We have better measuring equipment and we have better relationships with airlines so it should be much better but we're still at the hands of both the weather and the volcano; those are the two uncertainties."
Champions League finalists Barcelona are already considering bringing forward their flight to London ahead of Saturday's final at Wembley against Manchester United.
The Catalan club had originally planned to travel on Thursday.
"Let's see what they [the experts] tell us and if they say we shouldn't risk it we'll travel tomorrow or the day after," said Barcelona coach Pep Guardiola.
During last April's six-day shutdown only a handful of flights took off or landed in the UK.
Thousands of Britons found themselves stranded overseas forcing many to make long and expensive journeys home by land. Airlines estimated the shutdown cost them $1.7bn (£1.1bn).
The CAA said ash levels would now be graded as low, medium or high, and airlines would be notified if levels reached medium or high.
Airlines would then consider whether to fly, according to risk assessments already carried out, the CAA added.
The Foreign Office is advising passengers to remain in regular contact with their travel agent or airline for the latest news on the status of flights and bookings.
The Grimsvotn volcano in Vatnajokull National Park began erupting on Saturday with ash rising to 20km (12 miles) but, although still active, is now not as powerful with a plume of 13km (8 miles).
Iceland's airspace has been closed for a period as a result.
Ash from the volcano, which is 60 miles (97km) from the nearest human settlements, has settled over farmland and livestock, causing difficulties for some farmers and tourists have been evacuated from the country's main national parks.
The Grimsvotn volcano lies beneath the ice of the uninhabited Vatnajokull glacier in south-east Iceland. The latest eruption is its most powerful eruption in 100 years.
Experts say this 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.