An erupting volcano in Iceland is disrupting air traffic across the U.K. and northern Europe on Thursday, leaving tens of thousands of passengers stranded.

Ash clouds drifting from the spewing volcano have forced airlines to cancel flights in Britain, Ireland and the Nordic countries.

The British airport authority cancelled all flights for the remainder of Thursday to and from Heathrow Airport in London, Europe's busiest airport. Another 138 flights were cancelled at Gatwick, Britain's second-busiest terminal in London.

Britain's Civil Aviation Authority said non-emergency flights would be banned in all airports until at least 6 p.m. (1700 GMT, 1 p.m. EDT).

Aviation officials say "volcanic ash represents a significant safety threat to aircraft" because it can get in the engine and disrupt them.

"Safety is our top priority," Kevin Briscoe, a spokesman for National Air Traffic Services, told Business Week. "At the moment there is no indication of how long it will take to clear."