The national rugby unions of eight of the sport's major nations released a statement on Tuesday warning players and support staff against joining the rebel R360 competition.

The R360 competition has backing from private investors and former England centre and 2003 Rugby World Cup winner Mike Tindall is the public face of the project, which is shrouded in secrecy.

Few details are known about the plans, but reports have emerged it would involve six to eight men's teams and four women's teams, and would hope to attract the best players from all over the world to play for the competition's franchises.

But there are concerns about where the competition would fit into an already congested calendar.

In response, the national rugby unions of Australia, New Zealand, South Africa, Ireland, England, Scotland, France and Italy said in a joint statement on Tuesday that players who joined the R360 project would make themselves ineligible for national team selection.

"As a group of national rugby unions, we are urging extreme caution for players and support staff considering joining the proposed R360 competition," they said.

"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."

The eight unions criticised the project for having given "no indication as to how it plans to manage player welfare".

Organisers of R360 were quick to release their own statement, playing down any fears about what impact its competition would have on players.

"Player welfare is one of the key reasons for creating our global series, which will greatly reduce player load and capture the attention of a new generation of fans globally," the statement said. (AFP)