A plume of ash from an erupting Icelandic volcano headed for Britain on Tuesday, forcing flight cancellations and a change in US President Barack Obama's travel plans.
The eruption of Grimsvotn has raised concerns over a repeat of last year's travel chaos sparked by the eruption of another Icelandic volcano which led to the biggest shutdown of European airspace since World War II.
"The low-level winds are... blowing strongly towards the UK," said Peitur Arason of the Icelandic Meteorological Office, as British air traffic control operator Nats said volcanic ash was expected to reach Scottish airspace by early Tuesday.
Obama left Ireland for Britain on Monday night, one day ahead of schedule because of fears that the ash cloud could affect travel on Tuesday. The Irish visit was the first stop on a six-day European tour.
The change to Obama's plans came shortly after a Scottish airline announced it was axing a handful of flights early Tuesday.
Glasgow-based regional airline Loganair, which serves mainly Scottish destinations, said it had axed 36 services.
Flagship carrier British Airways, Dutch airline KLM, Irish carrier Aer Lingus and budget airline Easyjet also suspended flights on late Monday destined for northern Britain.
Two days into its most powerful eruption in over a century, monitors said ash particles from the Grimsvotn volcano had been scattered across much of Iceland, forcing the country to close its airspace on Sunday.
But winds shifted and began pushing the ash to the south, allowing the North Atlantic island nation to reopen all of its four airports by Monday evening.
However, Hjordis Gudmundsdottir, a spokeswoman for Iceland's airport authority Isavia, pointed out that "with a volcano still erupting one can only imagine the possibilities of the airport being shut again. Any sort of predictions are impossible."
Denmark's autonomous territory Greenland, the only other place where airspace so far has been shut due to the ash, extended the closure of some airspace until at least midnight GMT.
"The main question tomorrow is if the (ash) production will affect Scotland or Northern Ireland," Gunnar Gudmundsson, a geophysicist colleague of Arason, told AFP, pointing out that there was also a danger the ash could get into a jetstream to the south of Iceland and "could head to the North Sea."
Gudmundsson stressed, "It is not clear how this (ash) production will continue."
"It's very difficult to guess what will happen," he said, adding the explosive, ash-producing phase of the eruption would hopefully end within a few days.
Experts also point out that the ash from the ongoing eruption appears coarser than the very fine ash from last year's blast, and should therefore not travel as far.
European air traffic controllers in Brussels said Monday they did not expect any further airspace closures due to the ash until the end of Tuesday.
A spokeswoman for EU transport commissioner Siim Kallas, however, suggested that the picture was less certain.
"There is at the moment a possibility of volcanic ash affecting the European airspace starting with the northwestern areas like the UK and Ireland possibly today or tomorrow," she said.
During last year's eruption of the neighbouring Eyjafjoell volcano, more than 100,000 flights were cancelled and eight million passengers stranded, dealing a harsh blow to the airline industry, particularly in Europe.
The threat of a repeat sent airline shares across the continent tumbling on Monday, with German Lufthansa, Air France-KLM, British Airways and Scandinavian airline SAS all seeing falls of around three to four percent.
How far the ash travels will depend on the strength of the winds and the intensity of the eruption, experts agree, pointing out that historically Grimsvotn eruptions have tended to have very brief explosive stages, with the intensity usually subsiding significantly within a few days.
However Gudmundsson pointed out, "this is a much bigger eruption than the recent ones at this volcano," which is Iceland's most active - having erupted nine times between 1922 and 2004.
In fact, it is the most powerful eruption in more than a century from the volcano - located at the heart of the country's biggest glacier, Vatnajoekull in southeastern Iceland - with its plume initially reaching a height of 20 kilometres (12 miles).
On Monday, the plume stood at around 10 kilometres, slightly above the peak of last year's eruption ash column from Eyjafjoell.
"We hope the ash plume will be lower tomorrow.... I think it will decline more and more," Gudmundsson said, while adding that "there are still many open questions."
Meanwhile, football club Barcelona said it would make a decision Tuesday regarding travel plans for the Champions League final which they are due to contest with Manchester United in London on Saturday.
British transport minister Philip Hammond said that there had been some delays to flights but added Britain was better prepared after last year's travel chaos.