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Clouds of ash from an erupted Icelandic volcano drifted to parts of Europe on Thursday, forcing air-traffic controllers to close airspace to commercial aircraft and compelling airlines to cancel thousands of flights until Friday morning at the earliest.

Airspace in the U.K., Ireland, France, Belgium, the Netherlands, Norway, Denmark, Sweden and Finland was affected, and the havoc threatened to spread further with Germany also reviewing the situation as the clouds were sent south by the prevailing winds. In London, Britain’s air traffic service extended the ban on most air traffic in England until 1 p.m. London time (8 a.m. Eastern time) Friday, but flights to Scotland and Northern Ireland may be allowed to resume earlier, the Associated Press reported.

Clouds of ash from an erupted Icelandic volcano drifted to parts of Europe on Thursday, forcing air-traffic controllers to close airspace to commercial aircraft and compelling airlines to cancel thousands of flights until Friday morning at the earliest.

Airspace in the U.K., Ireland, France, Belgium, the Netherlands, Norway, Denmark, Sweden and Finland was affected, and the havoc threatened to spread further with Germany also reviewing the situation as the clouds were sent south by the prevailing winds. In London, Britain’s air traffic service extended the ban on most air traffic in England until 1 p.m. London time (8 a.m. Eastern time) Friday, but flights to Scotland and Northern Ireland may be allowed to resume earlier, the Associated Press reported.

Airlines across the world were hit by the disruptions, with Eurocontrol, a 38-country agency in Brussels that coordinates European flights, saying that roughly 3,000 of Europe’s 28,000 daily flights had been grounded. Trans-Atlantic flights between Europe and North America, which usually travel close to Iceland because jet-stream paths boost fuel economy, were disrupted, although some flights had been routed south to avoid the ash clouds.

Continental Airlines Inc. cancelled its morning departure from New York to London, though most of its evening flights to Europe remained on the schedule. UAL Corp.’s United Airlines cancelled its morning and evening flights from Washington to London, while Delta Air Lines Inc. cancelled its evening departure from New York to London.

FedEx Corp. said it hadn’t cancelled any flights, though some services had been diverted, forcing the shipper to truck packages to their final destinations.

Hundreds of thousands of travelers were likely to be impacted. In Ireland alone, about 60,000 passengers may be affected, the Dublin Airport Authority said. Russian Prime Minister Vladimir Putin had to postpone a trip from Moscow to Russia’s far northern region of Murmansk Thursday due to the ash cloud.

Volcanic ash represents a significant safety threat to aircraft, as particles reduce visibility, damage aircraft windshields and can cause aircraft engines to shut down.

On Wednesday, a volcano under Iceland’s Eyjafjallajokull glacier erupted for the second time in a month, spewing clouds up to 30,000 feet. Aircraft usually travel at altitudes up to 35,000 feet.

Experts say the Icelandic eruptions could continue on-and-off for months, potentially leading to continued delays and closures. An expedition team that returned from the volcano Thursday said the eruption was still going strong, with a lot of ash being propelled into the sky. “We are most likely going to have an ash cloud for the next couple of days at least,” said Oli Aranson, a meteorologist at Iceland’s national meteorological office. He added that the wind would continue to flow toward the British Isles until the middle of next week.

U.K. and Irish commercial airspace closed at noon (7 a.m. ET), and would remain closed until at least 6 a.m. Friday, authorities said. Britain’s Civil Aviation Authority said U.K. airspace hadn’t been affected as badly since World War II.

Authorities in Sweden said its airspace would be closed to turbine-engined aircraft from 10 p.m. local time (4 p.m. ET), after partial closures earlier in the day. Airspace above the Netherlands was being progressively closed and was expected to be completely closed by 7 p.m. Airspace controlled by Norway, Denmark and Finland also was completely or partially closed. Paris’ two main airports were due to shut by 11 p.m. at the latest, and others in the north of France started shutting at 5 p.m., aviation officials said.

BAA Ltd., which owns six airports in the U.K., said all domestic flights had been cancelled between its airports. In total, some 550 of the 1,250 departures and landings at London’s Heathrow, the world’s busiest international passenger airport, were cancelled Thursday.

A spokesman for BAA, a unit of Spain’s Grupo Ferrovial SA, said any flights to the U.K. from the U.S. that hadn’t departed by 7 a.m. local time wouldn’t be allowed to take off. Those that were due to land after noon were likely to be diverted to other airports.

British Airways PLC, the biggest operator at Heathrow by number of takeoff and landing slots, said it wouldn’t operate from London after 11:30 a.m. until further notice, and will be providing hotel accommodation overnight for stranded passengers.

U.K. domestic carrier Flybe was forced to ground 71 aircraft, with only 11 of its 565 daily scheduled flights operating by 9:30 a.m.

The Irish Aviation Authority said the country’s eastern airspace would be closed from noon to 8 p.m. The Dublin Airport Authority said about 60,000 passengers may be affected. Irish carrier Aer Lingus PLC said there would be major disruption to its flights from Dublin, Cork, Shannon, Belfast and London’s Gatwick, but trans-Atlantic flights were still operating.

Europe’s largest budget carrier, Ryanair Holdings PLC, canceled flights from 9 a.m. to and from the U.K., while no flights to and from Ireland operated from 10 a.m. Ryanair said it wouldn’t operate flights from 11:30 a.m. to and from Norway, Sweden and Denmark, and will close routes to and from Belgium from 3 p.m. It added that it would update its Web site later Thursday with flight cancellations for Friday.

Meanwhile in Iceland, emergency officials in Iceland were evacuating 700 people as new flash floods endangered farms in the area near the volcano, AP reported.

—Paul Sonne in London, Doug Cameron in Chicago and Dan Michaels in New York contributed to this article.

Write to Kaveri Niththyananthan at kaveri.niththyananthan@dowjones.com

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