The Australian Rugby Union has announced its intention to drop Perth-based Western Force from Super Rugby, although a legal challenge to the decision is being considered.
The ARU is reducing the size of the competition from 18 teams to 15 because of falling revenues and fan interest.
South Africa are to lose two sides in 2018 with Australia losing one.
The decision to drop Western Force has been "guided primarily by financial outcomes", the ARU said.
"This is a sad day for rugby, especially for Western Force fans," ARU chairman Cameron Clyne said.
"We accept that there will be anger and resentment over this decision and we sympathise with those fans. We sincerely hope that they are not lost to the game forever."
Rugby Western Australia plans to pursue "every possible means" to ensure the Force stay in the competition.
The organisation said in a statement: "RugbyWA is considering all options including bringing urgent proceedings in the Supreme Court and legal action relating to the circumstances which led it to enter into the alliance agreement with the ARU."
The Super Rugby competition, which features sides from Australia, New Zealand, South Africa, Argentina and Japan, initially expanded to 15 teams from 2011 before adding three further sides in 2016.
South African sides Cheetahs and Kings will now play in the Pro14.