Airlines grounded about 500 flights Tuesday after ash from an Icelandic volcano swept over Britain and towards northern Europe in a fresh travel nightmare for thousands of passengers.

Barely a year after a similar eruption in Iceland forced the biggest closure of European airspace since World War II, ash-laden clouds caused flights in and out of Scotland, northern England and Northern Ireland to be cancelled.

British Airways led the way in cancelling flights as the plume from the Grimsvoetn volcano spread eastwards, followed by Dutch airline KLM, Ireland's Aer Lingus and budget airlines easyJet and Ryanair.

"Approximately 500 flights were cancelled from the approximately 29,000 that would have been expected today across Europe," said a statement from Brussels-based air traffic controllers Eurocontrol.

The ash cloud caused minor air traffic disruption in Norway and closed a small part of Denmark's airspace on Tuesday, and Eurocontrol warned there was a "strong possibility" that would spread to southwest Sweden by Wednesday.

"This would have some impact on flights. However, given the new procedures in place and the predicted movement of the ash cloud over the coming days, the actual impact on flights is expected to be relatively low," it said.

Weather charts produced by the Volcanic Ash Advisory Centre run by Britain's Met Office predicted that a low cloud of ash would sweep across northern Europe overnight Tuesday, taking in northern Germany, Poland and Scandinavia.

Authorities say the ash can damage planes and stop engines, but Ryanair flew a plane through Scottish airspace and said it detected no ash on the aircraft.

It accused British and Irish authorities of over-reacting but reluctantly cancelled its flights to and from Scotland - almost 70 in total.

The growing chaos threatens planning for events ranging from the G8 summit to the Champions League final between Barcelona and Manchester United which takes place at London's Wembley Stadium on Saturday.

Barcelona said its squad would fly to London on Tuesday, two days earlier than planned, due to the "uncertainty" caused by the volcano.

European Union transport commissioner Siim Kallas played down fears that the situation could get as bad as 2010, when thousands of travellers from around the world were left stranded.

"We do not at this stage anticipate widespread airspace closure and prolonged disruption like we saw last year," Kallas told a news conference.

Iceland's Meteorological Office said activity at the volcano had slowed Tuesday and the ash plume had overnight dropped from its peak altitude of 20 kilometres (12 miles) to between three and five kilometres.

The most high-profile victim of the chaos was US President Barack Obama, who was forced to leave Ireland for London a day ahead of schedule on Monday night to avoid being stranded there.

Obama is among the leaders of the world's major industrialised nations due to attend a summit in northwest France from Thursday which could also be disrupted if the cloud drifts further south.

Latvian Prime Minister Valdis Dombrovskis was meanwhile forced to cancel a visit to Britain scheduled for Wednesday and Thursday, his spokesman said.

When Iceland's Eyjafjoell volcano erupted last year, the ash plume arrived first in Britain before spreading across the rest of Europe. Many airlines were deeply unhappy at the time at being forced to halt their flights.

Ryanair revealed Monday that last year's airspace shutdown cost it nearly 30 million euros (42 million dollars) and railed against the current closures.

It said its one-hour "verification" flight in Scotland's airspace on Tuesday showed no visible volcanic ash cloud.

"The absence of any volcanic ash in the atmosphere supports Ryanair's stated view that there is no safety threat to aircraft in this mythical 'red zone' which is another misguided invention by the UK Met Office and the CAA (Civil Aviation Authority)," the Irish airline said in a statement.

British transport minister Philip Hammond said the ash plume was a natural phenomenon "but the UK is now much better prepared to deal with an ash eruption than last year."

The CAA no longer issues blanket bans on flights; instead, it identifies areas of high, medium and low density ash and asks airlines wishing to operate in high or medium density ash to request permission to fly.

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