International rugby chiefs have until Friday to decide whether to back the Nations Championship planned for 2022.
World Rugby needs unanimous support from the 10 unions that make up Europe's Six Nations and the southern hemisphere's Rugby Championship.
While there are concerns about relegation and promotion, the Rugby Football Union broadly backs the idea.
"We support the concept and think it makes sense," RFU boss Bill Sweeney said.
In April, Six Nations unions agreed to a period of due diligence while weighing up offers from private equity.
"It's coming to the sharp end of that," added Sweeney. "It is well known publicly that June is a key month for where that is going.
"It's good for the global growth of the game. From a financial point of view, the numbers are good to enable us to reinvest back into the game.
"It is such a complex proposal. There are a number of issues around governance, around competition structure, and player welfare that we need to see a bit more detail on before we sign on the dotted line and I think that is the same with some of the other nations, so we are right in the middle of that process right now."
Scheduled to launch in 2022, the Nations Championship would see a top division of 12 teams from both hemispheres play each other once in a calendar year, either through traditional competitions like the Six Nations or the Rugby Championship, or in summer or autumn Test windows.
The top two teams would then meet in an end-of-year showpiece final.
A major sticking point is the concept of promotion and relegation, with Six Nations unions such as Scotland and Ireland yet to be convinced on the sustainability and vibrancy of the second division.
The proposals would also see November international schedules redrawn, with second-tier campaigners fearing the shake-up would reduce opportunities for smaller nations.
However, a 12-team first division would see regular exposure against the top nations for the likes of Fiji and Japan, who would each benefit from as many as 11 matches each year against top-tier nations.