Mental health issues cost the UK around £70bn every year, or 4.5% of GDP in lost productivity at work, benefit payments and health care expenditure.
According to the Organisation for Economic Co-operation and Development (OECD), better policies and practices by employers and the health system are needed to help people deal with mental health issues and get back to work.
The organisation's Mental Health and Work report also said that around one million claimants on Employment and Support Allowance (ESA), and as many on Jobseeker's Allowance (JSA) and other working-age benefits, have a mental disorder such as anxiety and depression that is hurting their prospects of finding work.
"Appropriate employment can be beneficial to wellbeing, so we agree that more specialist support is needed," said Paul Farmer, chief executive of mental health charity Mind
He added: "This will help tackle the barriers to entering and retaining work faced by people with mental health problems.
"The report also identifies the need for better integration between employers, the health service and the benefits system, as well as initiatives like Improving Access to Psychological Therapies (IAPT).
"Too many people currently fall through the gaps because these services are not joined up."
The OECD said acting early is the best way to prevent poor mental health leading to benefit dependency, both when people are still at work and early on during the sick-leave period.
The report found up to 370,000 Britons move onto disability benefit every year (1% of the working-age population), the highest rate in the developed world and twice the OECD average.
The leading cause for such benefit claims is mental illness, now accounting for around 40% of all new claims.
Farmer said: "Early intervention is key to supporting good mental health, and the new Health and Work service is an opportunity to help individuals get the right support at the right time."
"For the service to be effective, however, we need to see case managers with expertise in mental health who are able to recognise the impact a complex, fluctuating condition can have on somebody's ability to work."
The report follows the government's unveiling of the Health and Work service, which will enable employers to better manage sickness absence among their workforce.
The DWP claimed the scheme is expected to save employers £70m a year and cut the time people spend off work by 20% to 40%.
"As part of the government's long-term economic plan, we are taking action to improve get people back into work," said Mike Penning, the Minister of State for Work and Pensions.
He added: "This is a triple-win. It will mean more people with a job, reduced cost for business, and a more financially secure future for Britain."