London ? An enormous ash cloud from a remote Icelandic volcano caused the biggest flight disruption since the 2001 terrorist attacks as it drifted over northern Europe and stranded travelers on six continents. Officials said it could take days for the skies to become safe again in one of aviation’s most congested areas.

The cloud, floating miles above Earth and capable of knocking out jet engines, wrecked travel plans for tens of thousands of people Thursday, from tourists and business travelers to politicians and royals. They couldn’t see the source of their frustration — except indirectly, when the ash created vivid red and lavender sunsets.

Smoke and steam hangs Wednesday over the volcano under the Eyjafjallajokull glacier in Iceland, which has erupted for the second time in less than a month, melting ice, shooting smoke and steam into the air and forcing hundreds of people to flee rising floodwaters. Volcanic ash drifting across the Atlantic forced the cancellation of flights in Britain and disrupted air traffic across northern Europe, stranding thousands of passengers.

Nonemergency flights in Britain were canceled, and most will stay grounded until at least midday today.

Authorities in Ireland, Denmark, Norway, Sweden, Finland and Belgium also closed their air space. France shut down 24 airports, including the main hub of Charles de Gaulle in Paris; Germany’s Berlin and Hamburg were shut Thursday evening; and several flights out of the U.S. had to double back.

Kyla Evans, spokeswoman for air traffic service Eurocontrol, said half of all trans-Atlantic flights were expected to be canceled today.

At London’s Heathrow airport, normally one of the world’s busiest with more than 1,200 flights and 180,000 travelers a day, passengers stared forlornly at departure boards on which every flight was listed as canceled.

“We made it all the way to takeoff on the plane. … They even showed us the safety video,” said Sarah Davis, 29, a physiotherapist from Portsmouth in southern England who was hoping to fly to Los Angeles. “I’m upset. I only get so much vacation.”

A volcano beneath Iceland’s Eyjafjallajokull (ay-yah-FYAH’-plah-yer-kuh-duhl) glacier began erupting Wednesday for the second time in less than a month, triggering floods and shooting smoke and steam miles into the air. Video showed spectacular images of hot gases melting the thick ice, sending cascades of water thundering down the steep slopes of the volcano.

About 700 people from rural areas near the volcano were evacuated Thursday because of flash flooding, as water carrying icebergs the size of small houses rushed down the mountain. Most evacuees were allowed to return home after the floods subsided, but more flash floods are expected as long as the volcano keeps erupting, said Rognvaldur Olafsson of the Civil Protection Department.

The ash cloud became a menace to air travel as it drifted south and east toward northern Europe — including Britain, about 1,200 miles away.

The ash plume drifted at between 20,000 feet and 36,000 feet, where it could get sucked into airplane engines and cause them to shut down. The smoke and ash also could affect aircraft visibility.

Britain’s air traffic service said early today it was extending a ban on most air traffic until 7 p.m. local time today, but flights to Scotland and Northern Ireland, and North Atlantic flights to and from Glasgow, Prestwick and Belfast airports may be allowed until 1 p.m. local time.

The agency said Britain had not halted all flights in its space in living memory, although many were grounded after the Sept. 11, 2001, terrorist attacks in the United States.

“People can’t remember a time when it has been on this scale,” said Patrick Horwood of the air traffic service. “Certainly never involving a volcano.”

Eurocontrol spokeswoman Evans said the ash had led to the cancellation of about 4,000 flights within Europe Thursday, and that could rise to 6,000 today. Several U.S. flights bound for Heathrow, including those from Chicago, San Francisco, Denver, Las Vegas and New York, had to return to their departure cities or land elsewhere when London airports were closed.