Oneworld alliance member Finnair is formally seeking to participate in the joint business arrangement between British Airways and Japan Airlines.
Finnair intends to join the co-operation which will involve the three carriers sharing revenues and co-ordinating flight schedules on services between Europe and Japan.
BA and JAL started their joint arrangement in October 2012.
Finnair, which has built a strategic long-haul operation centred on Asian routes, says its entry into the pact will "strengthen" the business and enable the alliance to compete more effectively.
Chief executive Pekka Vauramo says Japan is one of the airline's "core markets".
Finnair operates from Helsinki to Tokyo, Osaka and Nagoya.
BA serves both Tokyo airports from London Heathrow while JAL operates from Tokyo to London and Paris, as well as from Tokyo to Frankfurt. JAL is also planning a Tokyo-Helsinki link from July.