Canada Life Group Insurance's Employee Assistance Programme (EAP) has revealed a fifth (21%) of calls made to the service in Q1 2015 concerned mental health - a 40% increase from Q1 2014.
While stress accounted for the largest proportion of these calls (43%) - and the number of calls regarding stress is up by 9% over the year - depression is the fastest growing mental health concern.
The number of employees seeking help for depression through the EAP EmployeeCare, attached to group income protection, was up by 63% compared to Q1 2013.
Depression now accounts for almost a third (31%) of mental health related calls.
Canada Life said while plenty of focus is given to stress in the workplace, little is dedicated to depression.
Yet the rate of increase in calls about this condition suggests depression may overtake stress as UK employees' biggest mental health issue.
The only mental health concern that has seen a reduction in calls is anxiety, with the number of calls relating to this falling by 11% in this period.
More than half (57%) of calls regarding mental health issues were work-related, and feedback from the Capita counselling team suggested an increased fear of redundancy is causing greater depression and anxiety among employees.
More than half (58%) of those who were not at work when they first requested counselling support had returned to work by the end of the counselling.
Before counselling, 51% of users with mental health concerns believed they were coping poorly with their jobs. After receiving counselling, this proportion fell to just 5%.
Paul Avis, marketing director, Canada Life Group Insurance said: "It is actually very positive news that only 1 in 5 of our calls to our EAP is mental health related, showing that our message- EAP is a work/life support service rather than a ‘you have to be broken' counselling service- is getting through.
"However, when people do call on mental health issues the results need to be understood. The negative impact of stress on staff has been well documented, some would say overly so, but the analysis of calls to our Employee Assistance Programme suggests that depression is nearly as prevalent among UK employees."
He added: "More importantly, the number of calls made about depression has shot up by 40% in the last year alone, and with fear of redundancy cited as a significant cause. It is clear that employers need to turn their attention to wider mental health issues, focussing not only on stress but also the effects of depression and anxiety.
"Employers have a ‘duty of care' responsibility to their staff to do as much as possible to prevent work-related mental health issues from developing. It is also vital that they provide appropriate support should an employee need it.