A global association of air traffic control companies added that the ash was likely to disrupt European air space for “several days”, the AFP news agency reported.
Europe’s air-traffic control centre predicted 17,000 flights would be cancelled on Friday, and warned travellers they may face further disruption again on Saturday.
Some 12 European nations have closed their air space or grounded flights with the plume of black smoke threatening plane engines and pilot visibility.
Earlier, experts cautioned the fallout from the volcano in southeast Iceland could take several days to clear, and aviation authorities have refused to say when the skies would clear again.
On Friday, Britain, Austria, the Czech Republic, Denmark, Lithuania, Latvia, Estonia, Belgium, Poland and the Netherlands shut down all or most of their airspace.
Finland, France, Germany, Russia and Spain experienced major airline disruption on Friday, although Sweden and Norway have begun to reopen airspace.
The chaos has also threatened to inflict heavy financial losses on airlines and businesses, and there are growing fears about the transportation of food supplies and essential goods, should the ash delay flights for longer.
Poland has said it may postpone Sunday’s funeral of Lech Kaczynski, the Polish president, who was killed in an air crashalong with 95 others last week, with the volcanic ash threatening to prevent some world leaders from attending.
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However Kaczynski’s family have urged that the state funeral go ahead as planned in the southern city of Krakow.
Volcanic eruptions are a much-feared peril in civil aviation, disgorging fine ash that can damage jet engines, clog fuel systems and drastically reduce visibility.
Tim Friend, Al Jazeera’s correspondent at London Heathrow, said passengers were becoming increasingly frustrated at the airport, where restrictions were in place at least until midnight on Friday.
“Some sources I’ve been speaking to have been indicating that this will probably be extended further,” he said. “There may be intermittent disruptions to flights for months to come.”
The cloud is drifting 6,000-11,000 metres up in the air and is not visible from the ground.
Icelandic airports have remained open, with winds blowing ash away from the island.
Everton Fox, Al Jazeera’s meteorologist, said the winds over Europe seemed to be moving from a western direction.
Volcanic ash spreads across northern Europe
“That should alleviate the problem in the southern half of the United Kingdom for example, but it will push the ash cloud further towards southern parts of Scandinavia, at least through the course of Saturday,” he said.
“But as we go into the second half of the weekend, those winds are coming from more a northern direction. So if anything, it may just take us back to square one.”
The volcano under the Eyjafjallajokull glacier erupted for the second time in less than a month on Wednesday, sending plumes of black ash and white steam into the air.
Ice chunks the size of houses tumbled down from the glacier on Thursday, as hot gases melted the ice.
Emergency officials were evacuating hundreds of people as flash floods threatened to endanger farms in the area but residents could later return to their homes after authorities said flood waters had decreased and appeared stable.
No people or animals have been harmed so far in the flooded rural area about 125km east of the capital, Reykjavik, but some farmland has been ruined.
In Iceland, the ash has so far mainly caused disruptions on the ground.
“We had to close roads because of the ash yesterday [Thursday], even though it was not very thick,” Kjartar Thorkelsson, a local police chief, said.
He said the toxic ash “is particularly dangerous for animals, since it can go into water and the grass they eat.
“It is not as dangerous to humans”.
The World Health Organisation has said Europeans should try to stay indoors if ash from the volcano starts raining down from the sky.