Most flights between Europe and North America will be delayed due to the spreading cloud of volcanic ash stretching across much of the northern Atlantic, the European flight control agency said.
Flights will have to be re-routed north over Greenland or south over Spain to avoid the 1,200-mile long cloud stretching from Iceland to northern Spain, Eurocontrol said. This will increase flying times by about an hour in either direction.
"We assume that basically most of the transatlantic flights will have to be re-routed on Saturday," Eurocontrol spokeswoman Kyla Evans said. "We expect substantial delays because of that."
Approximately 600 planes make the oceanic crossing every day. Around 40% will be re-routed southward and the rest will skirt Iceland from the north.
The plume of ash also forced the closure of 15 airports in northern Spain and is expected to expand into southern France during the day, carried along by Atlantic winds. Spain's main international airports of Madrid and Barcelona were expected to remain open.
Just over 100 flights were cancelled at mainland Portugal's three international airports because of the ash cloud today, the national airport authority ANA said.
"During the day, the area affected by volcanic ash is expected to extend from Iceland, south to Portugal and possibly as far east as Barcelona and Marseille," a Eurocontrol advisory said.
Until Eyjafjallajokul, the volcano in southern Iceland, stops its emissions, the key to the future course of Europe's ash crisis will be the prevailing winds. The eruption of the glacier-capped volcano has shown no signs of stopping since it began belching ash on April 13. It last erupted from 1821 to 1823.
Since the ash is reaching altitudes of up to 35,000ft, right in the path of most transatlantic flights, it will effectively block the usual routes. Eurocontrol said this would cause significant congestion, particularly in the airspace over Spain and Portugal where many of the diverted flights are heading.
A transatlantic flight from New York to Paris, which is normally about 3,600 miles, could add on another 600 miles - and an hour of flying time - by being diverted over the Iberian Peninsula.