Spanish firm Gómez-Acebo & Pombo and Portuguese ally Vieira de Almeida & Associados have split following a review of both firms' international strategies.
It is understood that Gómez-Acebo wanted to have its own representative office in Portugal. The firm has a new international strategy and has already announced the opening of a London office in September.
Vieira wanted to pursue relationships with a variety of firms in Spain, both national and international, and took a decision to distance itself from Gómez-Acebo.
Vieira released a statement saying that the two firms had "decided to end their preferential cooperation agreement as a consequence of the evolution of their respective markets and each firm's own strategic plans".
A spokesperson for Vieira said: "We have very good relationships with other players in Spain, so this is not a problem for us.
"We work with the majority of Spanish firms in our country and there are international firms in Spain that are sending us work.
"There's no longer any sense in being a non-exclusive partner."