Managing director Rupert Loman said that GAMEfest, which will run days before the Eurogamer Expo begins, will "burden" exhibitors and possibly split the market.
"We don't feel it's going to affect our attendance, and from what we've heard it isn't impacting what we will have exhibiting, or the quality of our show," he told MCV.
"Many [exhibitors] are saying they will do the GAME show too, not instead of. But they face a challenge to do two events in one week, and having to increase their budget - history has shown us that isn't successful for anyone."
Meanwhile, one publishing source called the move a "huge burden" and said having to dedicate resources to both shows is "an impossible decision".
"We previously supported Eurogamer Expo, but we can't not support the retail market leader," they said.
"So effectively we have to build a separate stand for the public, or separate stands for both shows, or pay extra to quickly move from Birmingham to London. It's an impossible decision."
Announced at the Video Game BAFTAs earlier this month, GAMEfest will held at Birmingham's NEC from September 16 to 18.
Meanwhile, the London-based Eurogamer Expo will run from September 22 to 25. Early entry tickets to the event have already sold out.