Delta purchased the stake to gain access to the lucrative London market. The European Commission and the U.S Department of Justice signed off last week on the transaction.
Starting July 3, the airlines will jointly market flights on 108 routes. Fliers can earn and redeem miles on each carrier. Each airline will honor the elite status and lounge privileges of the other's frequent fliers.
The two airlines hope to start sharing revenue on trans-Atlantic flights sometime in 2014.
Virgin CEO Craig Kreeger said his airline is still considering whether it will join the larger SkyTeam alliance anchored by Delta.
Copyright 2013 The Associated Press.