Sir Richard Branson is to sell his majority ownership of Virgin Atlantic, the airline has announced.
Air France-KLM will acquire a 31% stake in the airline currently held by the billionaire entrepreneur's Virgin Group for £220 million.
Delta will retain its 49% share of the company, leaving Virgin Group with 20%.
The transactions are subject to a series of agreements and approvals, Virgin Atlantic said.
The carrier will remain a "UK airline with a UK operating certificate" and will continue flying under the Virgin brand, according to the statement.
Sir Richard said: "Virgin Atlantic has made a big difference to people's flying experience over the past 33 years and transformed the airline industry for the better.
"This is a fantastic opportunity to extend our network and create a stronger customer champion, as well as being extremely beneficial to our people and the Virgin Atlantic brand that our customers love dearly."
Air France-KLM, Delta and Virgin Atlantic marked the announcement by unveiling a plan to create "the most comprehensive transatlantic route network".
They pledged to offer convenient flight schedules with competitive fares and reciprocal frequent flyer benefits, including the ability to earn and redeem air miles across all carriers.
The measures include the "co-location of facilities at key airport hubs" to improve connectivity times and allow premium passengers to access each airlines' lounges.
Virgin Atlantic chief executive Craig Kreeger said: "We built the Virgin Atlantic brand by providing customers with the choice they deserve and a travel experience they love.
"We couldn't be more excited that the next stage of our growth will be at the heart of the strongest partnership for customers travelling between Europe and North America.
"Together with our friends at Air France-KLM and Delta we will build on this vision of our teams creating irresistible experiences for customers flying on our network."
Air France-KLM said the partnership will provide passengers with "even more choice", while Delta claimed the deal would "spur additional benefits for customers, employees and shareholders".
Virgin Atlantic offers UK flights from Heathrow, Gatwick, Manchester, Glasgow and Belfast - with destinations mostly in the US and the Caribbean.
There is growing competition among transatlantic airlines with carriers such as Norwegian, WestJet and Level offering low cost long haul flights.
In a letter to Virgin Atlantic staff, Sir Richard wrote that the airline has "made a massive difference to people's flying experience and changed the airline industry for the better".
But he bemoaned the lack of slots at Heathrow and Gatwick, saying this is stopping the company from enjoying a feed from Europe or providing extra onward journeys to passengers it is carrying to London.
Addressing the decision to sell his majority stake and form a wider partnership, he explained that he wanted to be certain "all the necessary building blocks are in place for Virgin Atlantic to continue to prosper and grow for the next 50 years".
He also tweeted a quote which read: "No one can whistle a symphony. It takes a whole orchestra to play it."
Aviation consultant John Strickland described the announcement as "an unexpected and surprising move by AirFrance-KLM".
He told the Press Association: "In terms of influence in the important Heathrow and North Atlantic market, this ticks all the boxes - and with Sir Richard Branson moving to a minority position, it will allow potentially a realignment of usage of Virgin's Heathrow slot portfolio."
Mr Strickland predicted that the move would "sharpen competition" with British Airways' parent company IAG.