Virgin Atlantic and Delta Air Lines plan a wide-ranging codeshare across 108 routes including lucrative transatlantic services from Heathrow.
The planned transatlantic joint venture by the two carriers came closer to fruition after Delta confirmed the completion of its purchase of 49% of Virgin Atlantic.
Clearance is still required from the US Department of Transportation Department.
The partner airlines expect that approval to come later this year, clearing the way for to co-ordination of timetables to start in the first quarter of 2014.
Once implemented, this will deliver further “significant additional consumer benefits and vibrant competition” to the transatlantic market, the carriers said.
“In the future, corporate and travel agency customers of both airlines will also benefit from an aligned sales effort on both sides of the Atlantic,” a joint statement said.
As a prelude, the two airlines plan to start cross-selling seats on each other’s flights from July 3 in a bid to better compete with the British Airways-American Airlines alliance across the North Atlantic. Flights will be bookable from Saturday (June 29).
The codeshare will enable each airline's passengers access to the other's loyalty schemes and lounges.
A combined 23 weekday flights will be available to the US from Heathrow – nine to New York, and direct daily services to Los Angeles, San Francisco, Chicago and Washington, with Seattle likely to follow.
Virgin Atlantic will place its code on 91 Delta routes and the US carrier will place its code on 17 Virgin Atlantic routes, including the recently launched Little Red UK domestic services connecting London to Manchester, Edinburgh and Aberdeen.
This follows the US Department of Justice and European Commission approving Delta's purchase of the Virgin shareholding last week.
Atlanta-based Delta announced in December it would buy the 49% stake in Virgin Atlantic from Singapore Airlines for $360 million. Virgin founder Sir Richard Branson will keep his 51% share.
Virgin Atlantic chief executive Craig Kreeger said: "As new shareholders in the airline, Delta is an important ally in the all-important transatlantic market.
“We can stand firm together against the competition and can now offer more destinations, a smoother connecting airport experience and ultimately the best transatlantic on board experience.
“Virgin Atlantic's award-winning customer experience is loved around the world and we look forward to sharing our unique Clubhouse and in-flight hospitality with many Delta passengers over the years."
Delta president Ed Bastian said: "This is a strategic investment that enhances Delta's network with greatly improved access to London Heathrow, the leading marketplace on the transatlantic.
"As we unite two leading global airline brands in an innovative partnership that focuses on delivering enhanced benefits and services for our customers, we also become a more formidable competitor across the Atlantic."