Britain said flights could be disrupted from parts of the country today (24 May) by an ash cloud billowing from an Icelandic volcano, but said it did not expect a repeat of last year's travel chaos.
Britain's Met Office is predicting the plume of ash from the Grimsvötn volcano would cover the Irish Republic, Northern Ireland, Scotland and parts of northern Britain by 0600 GMT.
US President Barack Obama was due to fly into Britain on Tuesday from Ireland for a state visit, but was forced to change those plans and instead flew into London late last night.
The Irish Aviation Authority said flights to and from Ireland could be disrupted later in the week but did not expect problems in the next 48 hours. Iceland's main airport was set to reopen yesterday at 1900 GMT while other parts of Europe were on alert.
UK air traffic control company NATS said the eruption could affect Scotland from as early as 1700 GMT yesterday.
Last year, ash from an Icelandic volcano caused 100,000 flights to be cancelled, disrupting 10 million passengers and costing the industry an estimated $1.7 billion in lost revenues.
Asked if the ash cloud would cause some disruption to flights this time, a spokesman for Britain's Civil Aviation Authority said: "That's the way it's looking certainly at the moment."
Europe's air traffic control organisation said if volcanic emissions continued at the same rate the cloud could reach western French and northern Spanish airspace on Thursday.
President Nicolas Sarkozy is due to host Obama and other G8 leaders in France later this week.
Authorities have backed more relaxed rules on flying through ash after being criticised for being too strict last time.
"I think the regulators are a bit more sensible than they were last year," Michael O'Leary, chief of budget airline Ryanair, told a conference call. "We would be cautiously optimistic that they won't balls it up again this year."
Nevertheless, airline shares fell between 3-5%.
German Transport Minister Peter Ramsauer said he did not expect the eruption to disrupt air traffic to the same degree as last year, adding however there would be a flight ban for jet planes should particles from the ash cloud reach a higher concentration than two milligrammes per cubic metre.
Speaking to Sky News, British Transport Secretary Philip Hammond said authorities could work with airlines to "enable them to fly around concentrations of ash rather than having to impose a blanket closure".
Grimsvötn erupted on Saturday, with plumes of smoke shooting as high as 20 km (12 miles) into the sky. The eruption is the volcano's most powerful since 1873 and stronger than the volcano which caused trouble last year, but scientists say the type of ash being spat out is less easily dispersed and winds have so far been more favourable.
"The difference in impact on aviation comes down to three factors: the ash being produced by the eruption, the weather patterns blowing the ash around and new rules about planes flying into ash," University of Edinburgh volcanologist John Stevenson wrote on his blog.
Smothered in ash
But some were expecting problems. "It's too early to tell if Europe will be affected. What's certain is that when it is affected, there will be flight cancellations," French Transport Minister Thierry Marianai told Europe 1 radio.
Airlines as far away as Australia were monitoring the cloud. Norway's civil aviation body said the one or two flights a day to the Arctic archipelago of Svalbard would shut last night. A small part of Greenland's eastern airspace was also closed.
Iceland's aviation authority said it hoped it might be able to re-open the island's main airport by yesterday evening as the tower of smoke above the volcano appeared to have fallen.
The Icelandic Met Office said the plume from Grimsvötn, which last exploded in 2004, had fallen to just below 10 km (six miles) but that volcanic activity remained strong and steady.
The volcano lies under the Vatnajökull glacier in southeast Iceland, the largest glacier in Europe. People living in districts close by have been smothered in ash.
"[On Sunday] between two and three [in the afternoon] it brightened up a bit until eight in the evening, then it became black again," said Sigurlaugur Gislasson, 23, whose family owns a hotel near the town of Kirkjubaejarklaustur.
"It is like being in a sandstorm," he said. All the tourists who were staying at the hotel have gone, he added.