Airports across Europe have reopened and nearly all regularly scheduled flights are expected to resume Thursday, but travellers are still dealing with delays as airlines try to clear the massive passenger backlog caused by a cloud of volcanic ash.

An estimated 95,000 flights have been cancelled since the volcano under Iceland's Eyjafjallajokull glacier erupted last week, spewing out clouds of ash that drifted into European airspace.

Britain and most other European countries started reopening airspace Tuesday, clearing the way for thousands of stranded passengers to start making their way home.

"The crisis is petering out," said Brian Flynn, a spokesman for Eurocontrol, the European air traffic control agency.

Eurocontrol said it expected at least 22,500 flights to go ahead across Europe on Wednesday, and "almost 100 per cent" of flights to resume Thursday. A typical weekday sees about 28,000 flights, the agency said.

But even as planes head back to the skies, airline officials are warning passengers that it will take days — if not weeks — to clear the backlog and get operations back on schedule.

Duncan Dee, Air Canada's chief operating officer, said several flights between Toronto and Europe will go ahead as planned, but he said the airline's schedule is still not back to normal.

"In terms of the backlog itself, this is something that's going to take quite some time," Dee said.

Passengers with current tickets are being given priority, but Dee said the airline is trying to get stranded travellers rebooked as soon as possible. He advised all passengers to check the status of their flights online.

Foreign citizens whose trips or immigration to Canada have been flummoxed by the travel holdup are getting a reprieve, the Canadian government announced Wednesday. Visitors in Canada can get a free 30-day extension on their visa if it expired on or after April 15 and their departure was scuppered by the European airspace closure.

People who had planned to travel to Canada but couldn't make it can get a free replacement visa. And immigrants who weren't able to arrive in the country by their deadline can apply for relief, Immigration Minister Jason Kenney said.

Costly closures

The International Air Transport Association, which represents the airline industry, estimates the ash-induced travel disruption has cost the sector at least $1.7 billion US.

Giovanni Bisignani, the agency's CEO, said that at the height of the crisis, 1.2 million passengers a day were affected by the airspace closures.

"For an industry that lost $9.4 billion last year and was forecast to lose a further $2.8 billion in 2010, this crisis is devastating," he said.

Bisignani said it would take three years for the industry to recover from the losses of a week of cancelled flights, and he urged European governments to consider giving compensation to airlines.

A number of airlines have criticized the sweeping airspace closures imposed by European governments in response to the plume of ash, which can limit visibility and damage a plane's engines.

There is no internationally accepted standard explaining what level of ash concentration poses a threat to planes, Roberto Kobeh Gonzales, the head of the International Civil Aviation Authority, said Tuesday.

The ICAO, a United Nations agency responsible for aviation safety, provides data about volcanic ash clouds to individual governments, who in turn make decisions about airspace. On Tuesday, Gonzalez said the ICAO would work with scientists and the aviation authority to try and develop an international standard.

In Iceland, meanwhile, scientists said the Eyjafjallajokull volcano is still erupting, but the amount of ash billowing into the atmosphere was decreasing.

Scientists are also monitoring another volcano, Katla, which in the past has erupted in tandem with Eyjafjallajokull.

With files from The Associated Press