The first flight has landed at London's Heathrow Airport, Europe's busiest hub, since airspace across the continent was closed by ash spewed from a volcano in Iceland.
British Airways said it hoped about 24 other flights bound from the United States, Africa, and Asia would land later at the airport.
With so many flights having been canceled by the cloud of volcanic ash, it could take days, even weeks, to clear the backlog.
Britain's Civil Aviation Authority (CAA) emphasized that scientists and manufacturers had downgraded the risk of flying in areas of relatively low ash concentrations.
"The major barrier to resuming flight has been understanding tolerance levels of aircraft to ash. Manufacturers have now agreed increased tolerance levels in low ash-density areas," CAA head Deidre Hutton said.
Iceland's civil protection agency today said that the Eyjafjallajoekull volcano has lost nearly 80 percent of its intensity since its weekend peak.
Millions of travelers were stranded after European governments decided to close their airspace on April 15 following the volcano's eruption.
Experts had warned that volcanic ash could cause planes' engines to fail.
'Worse Than 9/11'
According to the International Air Transport Association (IATA), disruptions to European air travel from the volcanic ash cloud have cost the industry at least $1.7 billion.
IATA's chief executive, Giovanni Bisignani, said the scale of the disruption was worse than after the September 11, 2001 terrorist attacks in the United States.
On April 21, many European flights took to the skies as airports slowly reopened.
Meanwhile, the president of the International Civil Aviation Organization, Roberto Kobeh Gonzalez, said that it is safe to fly to Europe now that authorities there have decided to reopen airspace and traffic.
Problems remain with a backlog of flights and fears the volcano could spew more ash.
compiled from agency reports