The Hollywood Reporter said the information material that will be sent to SAG members will include an opposition statement.
The newspaper said union rules did not require the statement, but the SAG board decided to include it anyway.
SAG and the American Federation of Television and Radio Artists are in the home stretch of a deal to join forces in their representation of Hollywood performers. The AFTRA board votes on the proposal sometime this weekend.
The Reporter said it was not known if AFTRA would include an opposition statement in the materials sent to its members; however, the opposition to the merger appears strongest within SAG.