Airlines halted dozens of flights on Tuesday after a plume of ash from an erupting volcano in Iceland blew over Britain, even forcing US President Barack Obama to revise his travel plans.
Barely a year after a similar eruption in Iceland forced the biggest closure of European airspace since World War II, Britain's flagship carrier British Airways was the first to suspend flights from London to Scotland.
Dutch airline KLM, Irish carrier Aer Lingus and budget liner Easyjet then followed suit while some flights into the northeastern English city of Newcastle were cancelled.
Forecasters warned that the plume could reach the European mainland later in the week, threatening to disrupt planning for events ranging from the G8 summit to the Champion League final between Barcelona and Manchester United.
Low-budget airline Ryanair meanwhile said it would challenge advice from the Irish Aviation Authority (IAA) not to operate flights to Scotland until lunchtime.
"Following forecasts of significant volcanic ash in Scottish airspace, (we) have decided as a precaution that it will not operate any flights between London and Scotland on Tuesday ... that arrive in Scotland before 2:00 pm (1300 GMT) or depart from Scotland before 2:00 pm," a BA statement said.
"At present all other flights are unaffected," it added.
The most high-profile victim of the chaos was Obama who was forced to leave Ireland a day ahead of schedule on Monday night to avoid being stranded there.
Obama is among the leaders of the world's leading industrialised nations due to attend a summit in northern France from Thursday which could well be disrupted if the cloud goes further south.
Spanish football giants Barcelona said they would make a decision Tuesday regarding their travel plans for the Champions League final in London's Wembley Stadium on Saturday.
When an Icelandic volcano erupted last year, the plume of ash arrived in Scotland before spreading quickly across Britain, shutting down the whole country's airspace.
The ash then drifted across most of Europe, sparking the biggest shutdown of airspace in the post-war era.
Many airlines however were deeply unhappy at the time at being forced to halt their flights and the prospect of a fresh confrontation between carriers and aviation authorities loomed on Tuesday.
In a statement on its website, Ryanair said it strongly objected to an order by the Irish aviation authority (IAA) to halt flights, adding that "here is no basis for these flight cancellations and will be meeting with the IAA on Tuesday morning to have this restriction on Ryanair flights removed as a matter of urgency.
"Ryanair believe that there is no safety risk to aircraft on fights operating to and from Scotland and together with other airlines will be complaining to the Transport Minister and regulatory authorities about these latest and unnecessary cancellations."
British transport minister Philip Hammond said there had been some delays to flights but added Britain was better prepared after last year's travel chaos when Iceland's Eyjafjoell volcano caused major disruption.
"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," he said.
Britain's Civil Aviation Authority said that since last year's crisis it had brought in new measures, including a move that areas of high, medium and low density ash will be identified using data from the Met Office.
Instead of a blanket ban on flights, British airlines wishing to operate in high or medium density ash will now have to have a safety request approved by the CAA.
The request sets out measures airlines will take to reduce the risk of flying through ash.
"None has so far submitted a safety case to operate in high density ash," it said.
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