Ash from an Icelandic volcano forced the cancellation of dozens of flights to and from Scotland on Tuesday but Ireland's Ryanair said it would protest against "unnecessary" restrictions.
Worries about the effect of the ash cloud pushed forward U.S. President Barack Obama's planned departure from Ireland and he arrived on Monday night in Britain to begin a state visit.
Last year, ash from another Icelandic volcano caused 100,000 flights to be canceled, stranding 10 million passengers and costing the industry an estimated $1.7 billion in lost revenue.
Iceland's President Olafur Grimsson told CNN the latest eruption was on a "monumental" scale but would not wreak the same havoc on aviation.
"In terms of Europe, this is not going to be anywhere like what happened last year. It's a different eruption and I think Europe is also better prepared to deal with it," he said.
But airline shares fell between 3 to 5 percent on Monday as investors worried about the costs of canceled flights.
Norway's airport operator said the ash cloud would cause some flight restrictions on its west coast on Tuesday and Denmark said a small area of its airspace would be closed.
Asian airlines said they were operating flights to Europe as normal on Tuesday but said they were closely monitoring the volcano.
Barcelona soccer coach Pep Guardiola said fear of flight disruption could force his team to travel to London as early as Tuesday to prepare for Saturday's Champions League final against Manchester United. They had been due to fly on Thursday.
Ryanair, which listed 36 cancellations between Scotland and cities across Europe, said it had been told by the Irish Aviation Authority that it could not operate flights to Scottish airports until at least 1 p.m. (8 a.m. EDT).
"Ryanair believe that there is no safety risk to aircraft on flights 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," it said on its website.
The airline's pugnacious boss, Michael O'Leary, was a vocal critic of the restrictions imposed on airlines last year out of concern that ash particles could cause engine problems. He had said on Monday he was cautiously optimistic that regulators would not "balls it up again this year."
British Airways said it would not operate any flights between London and Scotland on Tuesday that arrived in Scotland before 8 a.m. EDT or departed from Scotland before that time.
Flybe, EasyJet and Aer Lingus all said they were cancelling some of their flights to and from Scotland on Tuesday.
Dutch airline operator KLM, part of Air France-KLM, said on Monday night it had canceled 16 flights flying to and departing from four British cities and scheduled for Tuesday. Fights to and from Aberdeen, Glasgow, Edinburgh and Newcastle would be canceled on Tuesday morning, it said.
Europe's air traffic control organization said that if the 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 criticized for being too strict last time.
German Transport Minister Peter Ramsauer said he did not expect the Icelandic eruption to disrupt air traffic to the same degree as last year. But he added that there would be a flight ban for jet aircraft should particles from the ash cloud reach a concentration higher than 2 milligrams per cubic meter.
The Grimsvotn volcano erupted on Saturday and smoke belched 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 that caused trouble last year.
But scientists say the type of ash being spat out is less easily dispersed and winds have been more favorable.