The chief executive officer of Ryanair Holdings Plc, Michael O’Leary plans to start a discount trans-Atlantic airline, offering fares as low as $12, following the “open skies” accord between America and European Union.

The new airline would fly from Ryanair’s existing bases including London Stansted, Dublin, and Frankfurt-Hahn, Mr. O’Leary said yesterday in a briefing to reporters. The carrier would go to secondary American airports at destinations including New York, San Francisco, San Diego, Boston, Dallas, and Florida.

The agreement to loosen rules on lucrative trans-Atlantic routes will make the new airline possible, Mr. O’Leary said. The new carrier would begin flights in three or four years and compete against network airlines including British Airways Plc and Virgin Atlantic Airways.

“Ryanair’s business model to date has been superb, but this could be a bridge too far,” an analyst at BGC Partners in London, David Buik, said. “He may well have to reconsider starting an airline in a market that’s already incredibly competitive.”

Shares of Ryanair, Europe’s largest discount airline, fell 5 cents, or 0.8%, to 6.01 euros in Dublin. The stock has risen 66% in the past 12 months, boosting the carrier’s market value to 9.3 billion euros.

Ryanair has grown from a single airplane in 1985 to a fleet of 133 flying to 130 European destinations. The Dublin-based airline is now the third-biggest in the world in terms of market value and competes with network carriers on short-haul routes.

Ryanair’s entry into the European market, as well as that of its biggest discount competitor, Easy-Jet Plc, triggered a price war among European airlines in the late 1990s.

Ryanair charges what it claims to be the lowest fares in Europe, with routes such as London to Barcelona regularly selling for as low as 1 penny, excluding taxes. Tickets on the new trans-Atlantic service will cost from 10 euros, Mr. O’Leary told reporters. The carrier will offer premium seats in addition to “no-frills” economy.

“The implications of a successful trans-Atlantic Ryanair would be significant,” a New York-based Merrill Lynch & Co. analyst, Michael Linenberg, said in a report. “It is not entirely clear whether the impact would be unequivocally bad.”

The proposed airline would push fares lower and might encourage other low-cost American carriers to move into the trans-Atlantic market, Mr. Linenberg said. Such airlines, including Southwest Airlines Co., JetBlue Airways Corp., and AirTran Holdings Inc., now fly only in North America and the Caribbean.

Discount airlines such as Ryanair and EasyJet have doubled their capacity in the past four years, leading network carriers to respond by focusing on attracting business- and first-class passengers.

“Consumers want frills included on long-haul flights, not ‘nofrills,'” a Virgin Atlantic spokesman, Paul Charles, said. “Opening up markets breeds fresh competition and new ideas, which really benefit consumers.”

British Airways, Europe’s third-biggest carrier, welcomed competition on all routes to America. “We are a full-service airline offering a world-leading product on-board and exceptional customer service,” Thomas Coops, a spokesman, said.