Manchester United and Bayern Munich fans are among supporters from across Europe who have accused their clubs of a "blatant power grab" over plans to radically reform the Champions League.
The clubs, also including Paris Saint-Germain and Arsenal, are part of the European Club Association (ECA), which meets on Friday to see if it can sign off on plans from governing body UEFA.
The proposals on the table would lead to a complete overhaul of how the group stage of Europe's premier club competition works from 2024.
Thirty-six teams -- up from the current 32 -- would all be placed into one giant pool and each play 10 games in a so-called "Swiss system" more commonly associated with chess.
At the heart of the reforms is the desire to play more matches.
The ECA's chairman, Andrea Agnelli, has described the proposed new format, in which each team would play at least four extra group-stage matches, as "close to ideal".
However, it is understood that some ECA members are still pushing for greater commercial control.
An open letter to Agnelli, signed by 17 fan groups from 14 clubs who are represented on the ECA board, said the voices of millions of supporters were being ignored.
"Your plans to restructure the Champions League by increasing the number of games, introducing qualification based on past achievements, and monopolising commercial rights present a serious threat to the entire game," it said.
"Instead of realising your supposed goal of 'building a successful, sustainable, and socially responsible football industry', you will only make the gap between the rich and the rest bigger, wreck domestic calendars, and expect fans to sacrifice yet more time and money."
It added, "Such a blatant power grab would be indefensible at the best of times, but at the height of a global pandemic, it is nothing more than crisis profiteering -- not to mention a stark contrast to the solidarity displayed by fans."
UEFA's executive committee is expected to make a final decision on the new Champions League format on Monday.