The breakaway league - set for an October 2026 start date - is hoping to attract the who's who of world rugby with big money.
However, the rugby unions of South Africa, New Zealand, France, Australia, Ireland, England, Scotland and Italy revealed that any player who featured in R360 would be ineligible for national duty.
"As a group of national rugby unions, we are urging extreme caution for players and support staff considering joining the proposed R360 competition," a joint statement from the nations read.
"Each of the national unions will be advising men's and women's players that participation in R360 would make them ineligible for international selection.
"We all welcome new investment and innovation in rugby, and support ideas that can help the game evolve and reach new audiences, but any new competition must strengthen the sport as a whole, not fragment or weaken it.
"R360 has given us no indication as to how it plans to manage player welfare, how players would fulfil their aspirations of representing their countries, and how the competition would coexist with the international and domestic calendars so painstakingly negotiated in recent years for both our men's and women's games.
"These are all issues that would have been much better discussed collaboratively, but those behind the proposed competition have not engaged with or met all unions to explain and better understand their business and operating model.
"The R360 model, as outlined publicly, rather appears designed to generate profits and return them to a very small elite, potentially hollowing out the investment that national unions and existing leagues make in community rugby, player development, and participation pathways."
The first player draft is set to be held in July 2026 for the inaugural season consisting of six male and four female teams.
By 2028, R360 aims to have 16 events running from April to September featuring franchise teams from London, Miami, Tokyo, Dubai, Boston, Cape Town, Lisbon and Madrid.