Delta Air Lines was hoping to restart flights next month from New York to Athens and Lisbon, two popular summer destinations, but it will probably have to wait a little longer. The European Union is planning to bar most Americans even as it welcomes travelers from more than a dozen other countries next week, dealing a blow to Delta and other airlines hoping to revive their business as travel across the Atlantic Ocean typically peaks.

International flights make up a minority of flights for U.S. airlines but are typically much more profitable than domestic ones. And flights to and from Europe are generally the most important. U.S. and European airlines had reduced the number of available seats on flights connecting the two markets by about 75 percent next month compared with last July, according to the aviation data provider OAG. A travel ban on Americans, which European Union officials confirmed on Friday, will probably lead to even deeper cuts.

“It’s a huge deal,” said John Grant, a senior analyst at OAG. “It is by far the jewel in the crown for many major airline networks, in terms of both revenue and profitability.”

Tens of millions of people flew between the United States and European Union countries in 2019. Many traveled for business to and from cities like New York, Los Angeles and San Francisco and Amsterdam, London, Paris and Frankfurt. Many others fanned out farther to vacation, particularly in the summer, when international flights are often nearly full as American families jet off to Italy and Greece, and Europeans check out New York and California.