Players are unlikely to embrace the proposed four-day Tests until administrators clarify how they plan to utilise the calendar space freed up by the move, the Federation of International Cricketers' Associations (Fica) said.
The International Cricket Council is set to discuss the idea of trimming the current five-day Tests by a day to ease a crammed international calendar and reduce player workload.
Fica, which counts players from England, Australia and South Africa as members among others, fears the new gaps in the calendar could well be filled with more cricket.
"From our discussions with players around the world, and our global survey data, it is clear that there is currently a lot of negative sentiment, within the global collective of players, towards such a significant change to the game's most traditional format," Fica said.
Four-day matches were given the green light by the ICC in 2017 when South Africa hosted Zimbabwe, and England have since played one against Ireland.
With an increasing number of Test matches ending prematurely, the administrators are keen to free up more space in the schedules for lucrative shorter-form matches.
Indian batting great Sachin Tendulkar told Reuters he is firmly opposed to shortening Test matches to four days from five and warned against straying too far from the game's roots in the quest to attract a younger audience.
"Given the obvious cricketing implications, if the ICC and/ or Boards do want to make a broader case for four-day Test cricket, we would need to clearly understand what both the economic and scheduling benefits would be, so we can discuss that with players and gauge genuine collective feedback," Fica said.
"It is particularly important for us, and the players, to understand how any additional calendar space in the playing schedule would be used.
"Making a fundamental change simply in order to provide calendar space to fill with additional or meaningless cricket is clearly not something we can support. Cricket's global structure desperately needs clarity, rather than further confusion."
England have said they will support the idea after 2023, Australia are to give serious consideration to the plan, while South Africa have advocated for four-day games to be played in the future.
The powerful Board of Control for Cricket in India (BCCI) has yet to offer an opinion, but India captain Virat Kohli said he was totally against the plan.
"Until such a time as we and the players are provided with the full picture and compelling reasons for change, we remain supportive of five-day test cricket, and would expect significant player resistance if a shift to that is imposed on players by the ICC and/or Boards," Fica said.