Global rugby chiefs face a deadline of Friday to back or shun World Rugby's Nations Championship plan.
The governing body has confirmed the Six Nations and Rugby Championship unions must deliver an answer on the radical proposals by the end of this working week.
World Rugby needs unanimous support from the 10 unions that make up Europe's Six Nations and the southern hemisphere Rugby Championship to progress its Nations Championship blueprint.
The proposed format would launch in 2022, with the Six Nations winners facing off with the top team from an expanded, six-team Rugby Championship.
World Rugby has confirmed end of business on Friday as the deadline for unions to show their support - or otherwise - for the Nations Championship, but opted not to comment further at this stage.
Final competition proposals would eventually face a vote before the full World Rugby council, but not without full support from the top 10 European and southern hemisphere nations.
The controversial plan has had its opponents ever since it was first floated by World Rugby.
The proposals include creating two-tier tournaments for both the Six Nations and Rugby Championship, including promotion and relegation.
The Six Nations unions have previously shown resistance to promotion and relegation, while leading players criticised World Rugby's lack of engagement over the entire issue back in March.
Pacific Islands campaigners fear the shake-up would reduce opportunities for Fiji, Samoa and Tonga, given the November internationals would be redrawn.
England rugby boss Bill Sweeney last week admitted decision time was looming on World Rugby's plans, and the new RFU chief executive also conceded there are still issues to be ironed out.
"It's coming to the sharp end of that," said Sweeney of the World Rugby plans. "It is well known publicly that June is a key month for where that is going.
"We support the concept. We think it makes sense. 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, around 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."