But many airline analysts say such dramatic discounts will be few and far between when the agreement goes into effect next March.
"There is no genuine reason I can see for a dramatic drop in fares on most routes," aviation industry consultant Dan Solon in Barcelona told The International Herald Tribune.
"Despite popular perception, the North Atlantic is already highly competitive," especially for leisure travelers, he said.
Britain's Civil Aviation Authority predicted passengers would save up to 10 percent from existing fares.
Under the terms of the deal, to be signed in Washington April 30, U.S. and European airlines will be allowed to fly from any European Union city to any U.S. city, and vice versa.
Ryanair, Europe's largest low-cost airline, said Thursday it would start an airline in 2010 selling economy-class trans-Atlantic seats for as little as $14 each way.
The new airline would serve five or six U.S. cities, Chief Executive Michael O'Leary told Flight International magazine.
Britain's Virgin Atlantic, Ireland's Aer Lingus and Houston's Continental Airlines also plan new service.