Volcano ash threatens Europe air travel

Still, authorities say they don't expect the kind of massive grounding of flights that followed last year's eruption of the Eyjafjallajokull volcano in Iceland because systems and procedures have been improved since then and the cloud is currently not expected to move over continental Europe.

Pilots unions, however, expressed concerns that the ash could still be dangerous.

Obama, who had been scheduled to spend last night in Ireland, was forced to fly to London early because of the ash cloud - he landed at the capital's Stansted Airport late yesterday. Last year's Icelandic eruption also forced a change in his schedule then, causing him to cancel a trip to Poland.

Many airlines said authorities last year overestimated the danger to planes and overreacted by closing airspace for five days amid fears that the abrasive ash could cause engines to stall.

CAA spokesman Jonathan Nicholson said authorities this time would give airlines information about the location and density of ash clouds. Any airline that wanted to fly would have to present a safety report to aviation authorities.

He said most British airlines had permission to fly through medium-density ash clouds.