More than 70% of managers questioned for the survey of 3,000 retail workers confessed to being "accidental managers" who had not planned to occupy management roles when they set out on their careers.
In addition, just under 60% said they had never received any formal management training.
The training deficit and resulting bad management was clearly having a negative impact on staff morale, CMI chief executive Ruth Spelman said.
Just under 50% of staff admitted to having left a job because of bad management, the survey showed, while 53% said they would take a pay cut to work with a better manager.
Spelmen said: "It's telling that the majority of individuals never set out to manage people, and have not been trained to do so.
"Government and employers need to address this worrying skills gap. In what other profession would it be acceptable for only a quarter of practitioners to hold some form of professional qualification?"