French clubs Racing 92 and Stade Francais have announced plans to merge from next season.
Both clubs, who play in the Top 14 league and are based in Paris, need their amalgamation to be ratified by the Ligue Nationale de Rugby (LNR).
In a joint statement, the clubs said the move needs to be made "to better cope with the challenges of performance and education".
Racing 92 are eighth in Top 14, while Stade Francais are 11th.
The extra place in next season's division will be filled by either a play-off or offering another promotion spot from the second-tier ProD2.
"This association, effective from next season, must give birth to a new club that will preserve the roots of Racing 92 and those of the Stade Francais Paris while multiplying the means put at the service of youth and French rugby," said the statement.
"Consolidated by its dual culture, by the fierce determination of its two presidents and its increased audience in a population pool unmatched in France, this new club has a long-term commitment to build a true reference day after day."
Analysis - it's like Tottenham and Arsenal merging
Chris Jones - BBC Radio 5 live rugby reporter
It's an utterly incredible story. It's unprecedented and bizarre.
These aren't two small rugby clubs struggling to make needs meet. They are two powerhouses of the French game.
It's the equivalent of perhaps, in England, Saracens and Harlequins merging or, in a football example, Tottenham and Arsenal merging.
The details are still a bit sketchy at the moment. One of the reasons could be that Paris is such a diverse place and it doesn't, perhaps, have a rugby identity like other parts of France.
Maybe they feel that in order to completely maximise their potential then there needs to be one superclub in Paris rather than two slightly smaller superclubs.