Companies have been underestimating the importance of their workers' mental health, a report by disability charity the Shaw Trust has said.
The pressures of office life are a cause of stress
A quarter of workers "will" experience mental ill health, the report said, but 80% of firms had no specific mental health policy.
Mental health problems cost firms £9bn last year in sick pay, the study found.
Moreover, workers with mental health issues who make it known face "widespread discrimination".
Cost to business
The charity said mental health was "probably the last workplace taboo".
Other issues surrounding employees' sex, age and race had all been largely addressed by firms, but mental health remained ignored, the charity said.
The situation was far worse than expected, said Sally Ward, people and policy manager with phone firm BT, which is a corporate partner of the trust.
Many people's understanding of mental health is limited, with people thinking of illnesses such as schizophrenia and bi-polar disorder, Ms Ward told the BBC.
"But a far larger number of people experience depression, stress, anxiety, or post-natal depression", she said.
"We need to support employees so they can do their work."
In addition to the £9bn firms spend on sick pay as a result of mental health problems, they also lose money due to lower productivity, the charity found.
A workforce with a sound mind is better able to work and therefore in a stronger position to enable business do its job, Ms Ward said.