Thousands of flights across Europe were cancelled Friday and airspace closed as an ash cloud from a volcano in Iceland continued to spread and move across the Continent.

The ash cloud, which limits visibility and can damage a plane's engines, will affect flights for at least the next 24 hours, the air traffic control body Eurocontrol said Friday.

The travel interruptions threaten to scuttle Prime Minister Stephen Harper's plans to travel to Poland this weekend.

The ash is spewing from a volcano beneath Iceland's Eyjafjallajokull glacier that erupted Wednesday for the second time in less than a month.

Eurocontrol said almost two-thirds of Europe's flights were cancelled Friday as airspace remained largely closed in Britain and across large chunks of north and central Europe.

"The skies are totally empty over northern Europe," said Brian Flynn, deputy head of Eurocontrol, adding "there will be some significant disruption of European air traffic tomorrow."

About 16,000 of Europe's usual 28,000 daily flights were cancelled Friday — twice as many as were cancelled a day earlier, Eurocontrol said. Only about 120 transatlantic flights reached European airports compared with 300 on a normal day, and about 60 flights between Asia and Europe were cancelled.

Montrealer David Bensadoun's flight from London's Heathrow airport was among them.

"My wife's pregnant and tomorrow is the ultrasound to find out if it's a boy or a girl," Bensadoun told CBC reporter Tom Parry. "I wanted to get home. I'll miss it."

The International Air Transport Association said the volcano was costing the industry at least $200 million a day.

Some flights resume

Flights through England aren't expected to resume until Saturday morning, but some flights have resumed in Scotland and Northern Ireland.

Airspace in parts of Sweden and Norway has been reopened, as well as in parts of France. But Germany has closed much of its airspace, as have Austria, Switzerland, Slovakia, Croatia and Hungary.

Poland expanded its no-fly zone to most of the country, excluding Krakow. Belgium extended its flight restrictions until late Saturday morning.

The closure of major international hubs like Heathrow Airport is having ripple effects around the world, forcing travellers to make alternate plans or wait until airspace is reopened. At Pearson International Airport in Toronto, at least 13 transatlantic flights were cancelled Friday morning.

Harper and political rivals Michael Ignatieff, Jack Layton and Gilles Duceppe were among those Canadians waiting to find out whether their travel plans would be thwarted.

Harper and a delegation of Canadian dignitaries, including the leaders of all three federal opposition parties, were scheduled to fly out of Ottawa early Saturday morning to attend the funeral of Polish President Lech Kaczynski and his wife, who died in a plane crash last weekend.

Denis Chagnon, a spokesman for International Civil Aviation Organization, said volcanic ash can do serious damage to planes.

"It can literally choke an engine, it can scratch the windows of a cockpit, it can almost take the paint off the aircraft," he said.

Chagnon said there is a system in place that allows aviation authorities to track volcano eruptions and alert pilots about the spread of the gritty, acidic ash.

Possible health hazard

The World Health Organization said Europeans should try to stay indoors if ash from Iceland's volcano starts coming down from the sky.

WHO spokesman David Epstein said the agency doesn't know the exact health risks from the ash cloud, but the microscopic ash is potentially dangerous for people if it starts to "settle" because inhaled particles can reach the lungs and cause respiratory problems.

"The erupted plume is at four to five kilometres in altitude and occasionally goes up to nine," said Armann Hoskuldsson, a volcanologist in Iceland.

Epstein said the health agency will keep monitoring the cloud's altitude and movement to try to determine if it poses a health risk.

With files from The Associated Press and The Canadian Press