Thousands of passengers were forced to abandon plans after ash from an Icelandic volcano caused the cancellation of 500 flights on Tuesday, but officials said Europe's airways would be less disrupted than a year ago.

As the eruption of Iceland's most active volcano began to run out of steam, a trail of ash from the plume which has been rising from Grimsvotn since Saturday drifted south into Scotland and parts of northern England, disrupting flights.

Among travellers affected were U.S. President Barack Obama, who left Ireland for Britain late on Monday ahead of schedule.

The Barcelona soccer team flew to London early for Saturday's Champions League final against Manchester United.

"At the moment we think Glasgow and Edinburgh will be affected throughout this afternoon but should be returning to normal tomorrow morning," British Transport Secretary Philip Hammond told BBC TV.

European air traffic agency Eurocontrol said around 500 flights had been canceled on Tuesday.

There were no immediate reports of threats to largest airports such as London Heathrow, but controllers said Denmark, Norway and Sweden could see some disruption by Wednesday.

Dutch airline KLM canceled two flights to Norway.

Over 10 million people were affected when a six-day shutdown of most European airspace halted over 100,000 flights in April last year. Airlines pegged their revenue losses at $1.7 billion.

Eurocontrol, the Brussels-based agency responsible for some of the world's busiest air corridors, said the impact on flights was expected to be far lower this time due to new procedures.

However there were fears of disarray caused by arguments over how to apply them, while sources told Reuters that a UK research plane designed to sample ash remained grounded for a second day in a wrangle over its deployment.

Officials say the new rules which allow airlines to fly in ash under certain conditions and better coordination by civil aviation bodies mean mass chaos will be avoided.

However, they are not accepted by all, with Germany backing a tougher stance for the sake of safety, aviation sources said.

Germany's meteorological service warned that the ash cloud could cause temporary airport closures in Hamburg and Bremen from midnight, and later possibly Berlin.

Ryanair has been particularly vocal and said it had flown a plane over parts of Scotland deemed to have high levels of ash and encountered no ash cloud. An inspection revealed no evidence of ash on the airframe, wings or engines.

"You have to ask why a combination of bureaucratic incompetence in the CAA (Civil Aviation Authority) and the Met Office last night shut the skies over Scotland when this morning we have now confirmed there is no volcanic ash material in the atmosphere over Scotland," Ryanair CEO Michael O'Leary told BBC television.

Technically, Britain and Ireland have ruled out closing their airspace for now but the new rules only give pilots discretion to fly through the worst-risk areas if airlines had submitted an approved safety case, which the CAA said none had.

WORST OF ERUPTION OVER

Britain's Met Office Volcanic Ash Advisory Center said its latest forecast showed no high density ash over Britain or Ireland early on Wednesday, and only a medium level over parts of Scotland. The forecast did show high density levels over parts of the northeastern Netherlands and northern Germany.

In Norway, helicopter services to North Sea oil operations faced problems. Denmark shut a small area of its airspace.

Eurocontrol has said that if the volcanic emissions continued at the same rate, the cloud could reach western French and northern Spanish airspace on Thursday.

French President Nicolas Sarkozy is due to host Obama and other G8 leaders in northern France this week.

Icelandic experts said the volcano had shown signs of calming. Met Office meteorologist Halldor Bjornsson said radar showed the ash plume was down to 5 km or less.

Seismic data measuring eruptions also showed a tailing off.

"It all fits in with the same picture that the worst of this eruption is over," he told Reuters. The ash currently being emitted was not enough to drift to European airspace, he added.

Grimsvotn erupted on Saturday and smoke belched as high as 20 km (12 miles) into the sky.

Bryndis Brandsdottir, a University of Iceland seismologist, said ash and other debris produced from the eruption had grown less intense and that based on past experience was likely to gradually abate in the coming days.

The eruption is the volcano's most powerful since 1873 and stronger than the volcano that caused trouble last year.