When a historic "open skies" agreement takes effect Sunday, London's Heathrow Airport will grab most of the attention, as Delta Air Lines and other carriers land there for the first time after decades of trying to gain access.

But the accord, approved 11 months ago by the United States and the European Union, is also a step toward a much broader overhaul of the flight rules governing the world's two biggest aviation markets.

Besides opening access to Heathrow, London's pre-eminent airport, the first phase of the open skies agreement will allow Europe's airlines to fly anywhere in the United States from anywhere in the European Union. Previously, Air France's jets, for instance, could only depart for the U.S. from Paris or one of its other French hubs.

"That's a major opportunity" for European carriers, said Delta Executive Vice President Glen Hauenstein. He said those airlines' new freedom to fly from any market in Europe to any market in the United States is Delta's major concern, and partly inspired Delta's decision to forge a joint venture with its longtime alliance partner, Air France.

As part of that joint venture, Air France plans to start flights from Heathrow to Los Angeles. Likewise, Delta's first flight to Heathrow departs from Atlanta tonight in time to land in London when the open skies agreement takes effect Sunday. Under the pact, the carriers will share revenues and profits from their combined trans-Atlantic flights, which are expected to generate $8 billion in revenues by 2010.

Under previous restrictions, only two British airlines and two U.S. airlines, American and United, could fly from the United States to Heathrow, one of the world's richest passenger markets. Under Delta's and Air France's joint venture, Air France supplied Delta with the much-coveted "slots" needed to fly to Heathrow.

Meanwhile, British Airways, another carrier that previously enjoyed an oasis at Heathrow, has announced plans to start a high-end airline this summer, OpenSkies Airline, that will shuttle business travelers between New York and Paris. Irish budget carrier Ryanair Holdings also has said it wants to eventually start trans-Atlantic service.

While flights to London account for about one in five passengers flying across the Atlantic, some experts predict that the other provisions of the open skies pact will ultimately boost all trans-Atlantic traffic, increase competition and drive down fares. Someday, a second phase of the agreement that has yet to be negotiated could lead to even more sweeping changes — global mergers such as a tie-up between Delta and Air France-KLM, which itself is a marriage of the French and Dutch carriers. The EU is seeking in the second round of open skies talks to loosen U.S. government restrictions on foreign ownership of U.S. airlines.

In a study prepared last year for the European Commission, consulting firm Booz Allen Hamilton estimated that the open skies agreement will increase trans-Atlantic traffic by one-third by 2011, to 37.5 million passengers annually. It should also help generate 72,000 new jobs, the London-based firm estimated.

But high fuel costs and the growing threat of a recession in the United States make such projections "very aggressive," said William Swelbar, a research engineer at the Massachusetts Institute of Technology's International Center for Air Transportation.

"With oil this high, are [airlines] really going to want to start new routes?" asked Swelbar.

At $106 a barrel, the cost of crude oil is now more than 70 percent higher than it was when the open skies pact was signed.

The cost of jet fuel, which is largely determined by the cost of crude oil, is the biggest expense of most U.S. airlines.

Airlines have responded to the rising costs by cutting expenses and capacity, raising fares and tacking on higher fuel surcharges, which can amount to almost $200 per passenger for some overseas flights. Delta earlier this month announced plans to cut 2,000 jobs and to cut domestic capacity 10 percent this year.

Swelbar said higher fuel costs also will likely mask the declines in fares that consumers might have hoped to gain from new competition at Heathrow and in other European markets.

Still, he said, adverse conditions now will only delay the impact of the open skies agreement. "Are we going to see some folks try some things? Certainly, we will," he said.