Over two-thirds of (69%) of senior business managers and owners don't believe suffering stress, anxiety or depression is serious enough to miss work, worrying research from AXA PPP has found.
Meanwhile, one in five managers revealed they would worry about the employee's capability to do their job if they had a mental health issues.
The research also found that one in six managers worry about the consequences for themselves, such as reflecting poorly on management style or having to do extra work.
One in four managers reported that they had experienced a mental health problem themselves, the same rate as for employees.
The survey questioned 1,000 employees and 1,000 managers and was conducted by OnePoll for AXA PPP found.
Highlighting the stigma that employees face, many said they would not disclose their condition to their employer.
Less than half (39%) of employees said they would tell the truth when calling in sick with stress anxiety or depression.
Some 15% were afraid of not being believed, 23% were afraid of being judged and 23% preferred to keep their health issues private.
Almost half (46%) of employees thought their employer didn't take mental health issues seriously while 12% of bosses thought their industry was affected by mental ill health.
Attitudes towards mental health issues in the workplace were thought to have improved in the last 15 years by 54% of employers while 30% said they had not seen any change.
Dr Mark Winwood, director of clinical psychology at AXA PPP healthcare, said: "Lack of understanding breeds fear so improving employees' awareness and understanding of mental illness is one of the most important things a company's senior management team can do.
"A critical first step is to challenge the stigma surrounding mental ill health in the workplace.
"Employers can begin by introducing a number of small but important changes such as promoting an open and honest culture where the facts about mental ill health are freely communicated and discussed."
'Employers are liable'
Turning awareness into action
Survey of 1000 shows
On International Day of People With Disabilities