"The airline industry needs adequate lead time to establish a plan for restarting air services, including scheduling aircraft and crews for these routes as well as for marketing and selling tickets," said the letter to the transport chiefs of both countries.

It was signed by the CEOs of American Airlines, Delta Air Lines, United Airlines, British Airways , Virgin Atlantic and JetBlue Airways.

Flights between the United States and Britain, normally some of the world's busiest international long-haul routes, are still largely stalled even as both countries are loosening COVID-19 restrictions and travelers are making summer holiday plans. Britain has administered the most vaccine doses per capita of any big country, and the United States is close behind.

MANAGING RISK

Willie Walsh, the former British Airways boss who now heads global airline body IATA, voiced exasperation over what he described as a failure by governments to "manage the net risk" proportionately.

"You see some politicians just afraid to make decisions because they're afraid of the consequences, whereas (in) the airline industry we live with that all the time," Walsh said at a virtual event on Wednesday. "Sometimes it's necessary for us to be vocal and try to force that approach upon people."

Since March 2020, the United States has barred nearly all non-U.S. citizens who have recently been in Britain. For its own citizens, the U.S. State Department on Monday eased its UK travel advisory, lowering it to a "Level 3: Reconsider Travel" rating.

Britain requires visitors from the United States to quarantine on arrival. Next week it is set to lift a nearly five-month ban on most international travel by its own residents, but it has so far excluded the United States from a small "green list" of destinations where British passengers can travel for leisure and return without a quarantine.

A spokesman for U.S. Transportation Secretary Pete Buttigieg noted that G7 Transport Ministers met last week "to discuss the complexities around reopening international travel and how to do so safely.

"These conversations are ongoing. The department will be reviewing the letter with other agencies as part of the whole of government approach to COVID recovery."

The British Embassy in Washington did not immediately comment.

A coalition of U.S. and European travel, airline, union, business and airport groups called last week for a full reopening of the U.S.-UK air travel market "as soon as safely possible."

Investment bank Cowen and Co said in a research note it believes "the Biden Administration needs to make a decision about international travel within the next week to 10 days...

"Americans are making their summer travel plans now, and without a decision on Europe soon, we believe they will continue to travel" to other destinations.

Travel firm TUI said on Wednesday it still expects a strong 2021 travel season, assuming some of its best destinations are added to Britain's "green list" later this month.

(Reporting by David Shepardson; Editing by Chizu Nomiyama, Carmel Crimmins and Peter Graff)

By David Shepardson