New Delhi, April 23 -- A jet fuel shortage is leading airlines to cut flights, and could cause a severe slowdown in travel this summer-particularly in Europe.

German carrier Lufthansa said this week it is canceling 20,000 flights from now through October, immediately reducing its flight schedule by 120 trips a day. Reductions are happening all over, including at the company's hubs in Frankfurt, Munich, Zurich, Vienna, Brussels, and Rome.

More than a dozen other airlines have also reduced capacity, including discount Norwegian airline Norse Atlantic, which cut flights from Los Angeles. American carriers are not in danger of running out of fuel, but U.S. airlines have begun to reduce flights to save money, too, with Delta Air Lines droppi...