British Airways, Finnair and Japan Airlines will start their planned joint business on flights between Japan and Europe next week.
The three Oneworld alliance members will share revenue and cooperate on scheduling and pricing from April 1.
The move follows Japanese regulators' approval last October for the BA and JAL to include Finnair in the joint business they established in 2012.
In February, the three carriers began selling joint fares on select routes for travel from next Tuesday.
JAL president Yoshiharu Ueki said the start of the joint business represents a "great milestone" in his airline's mid-term management plan.
He said: "Tighter cooperation with our Oneworld partners, BA and Finnair, will provide more seamless, consistent services and more travel choices for our customers."
According to the airlines, the benefits for passengers will include:
- The ability to mix and match flights on all three carriers,
- Online booking capability and check-in with any of the three airlines,
- Integrated customer support across the three airlines,
- Greater connectivity on all three airlines in case of disruption.
Finnair currently operates flights daily from Helsinki to Tokyo and daily in the summer (five times in the winter) to Osaka and Nagoya. JAL operates daily from Tokyo to Helsinki, Frankfurt, Paris and London, while BA operates daily flights from London to both Narita and Haneda airports in Tokyo.
Finnair CEO Pekka Vauramo said: "Together, the three of us will constitute the single largest player in Europe-Japan traffic, offering an extremely competitive customer proposition in this vital market."