Half of all group income protection (GIP) claims are for conditions where definitive medical evidence is difficult or even impossible to obtain, Canada Life has said.
In addition, the number of claims reported has risen dramatically over past years.
There was a 40% increase between 2008 and 2010 and claims reported for the year to date suggest this figure will continue to increase.
The group insurer said that, over the last year, 31% of claims made were for mental health issues and a further 20% for musculoskeletal conditions.
These conditions have consistently been the two most significant causes of claims for the last seven years.
Canada Life attributes this to the economic climate creating stressful working environments and putting more pressure on employees, eventually leading to more stress absence.
The insurer highlighted that this illustrated the need for employers to understand conditions where it is more difficult to obtain a clear cut diagnosis.
Paul Avis, Sales and Marketing Director at Canada Life Group Insurance, commented: "Subjective conditions, such as mental health issues and musculoskeletal conditions, can be more problematic for employers to understand and support.
"However as these figures show, they are the two leading causes behind Group Income Protection claims.
"Therefore it is important that employers consider how best to help their employees through the absence period and to support their rehabilitation and eventual return to work."
