Although there is legislation in place to protect them, workers who suffer from mental health problems are still the victims of discrimination, it has been claimed.
Speaking to Personnel Today, HR consultant Helen Waygood said the attitudes of many firms are rooted in the past.
The expert, who discovered she suffers from bipolar disorder twenty years ago, kept her condition a secret for much of her career.
Previously known as manic depression, it affects sufferers' moods and means they have episodes of depression and mania.
Ms Waygood stated: "For many employers, it's still far easier to marginalise colleagues with mental illness than face up to how it can be managed."
She added that until employees feel they can talk about their conditions without being stigmatised, they will continue to "suffer in silence".
Many HR professionals, the expert went on to suggest, will not admit this is a widespread problem in UK firms.