Germany's specialist Auto Motor und Sport said the Enstone based team could file the protest in Shanghai on Thursday, following stalemate meetings last week between technical directors and the FIA's Charlie Whiting.
The report said Red Bull has gone "surprisingly quiet" on the F-duct issue, "probably because they have been working on their own system".
"Even Sauber's engineers are thinking about how to integrate the system on their car," wrote Michael Schmidt from Shanghai.
The German newspaper Bild asked Mercedes' Norbert Haug about the rumors of a protest.
"It is well known that Charlie Whiting has said our system is legal. We voluntarily showed it to the FIA before the season started and got their formal go-ahead," answered the German.
Nonetheless, Bild said Mercedes' lawyers have been briefed and are ready to tackle an official protest.
"The rules say that anyone can protest, but we have the confirmation of the regulator FIA that our system is fully compliant," Bild insisted.
The newspaper story also reported that Red Bull is working on a similar system that could be ready in "a few races".