The report, Mental Health: We're Ready to Talk, highlights the sheer scale of mental illness in the UK and how much it costs the UK economy.
It is being launched at Business in the Community's (BITC) Responsible Business Week alongside a new campaign to remove the stigma of mental health and turn it into a priority boardroom issue.
The report suggests that 15.2 million work days were lost to mental illness in 2013, up from 11.8 million in 2010, at a cost to the UK economy of £70 billion, or 4.5% of GDP. In the UK there are 30.2 million workers, which means the number of days lost to mental illness is half the size of the working population.
One in six employees reported experiencing mental health issues, but less than half felt they could speak openly with their managers about it.
Some of the UK's biggest companies are joining the Workwell Mental Health Champions Group, including BT, Bupa, RBS, Mars and Proctor & Gamble.
Patrick Watt, corporate director of Bupa Health Funding and chair of the Workwell Mental Health Champions Group, said all areas of business must start to take mental health seriously.
“This report gives the strongest evidence yet of the business costs of ignoring mental health," he said. "It is a critical business issue and it must be promoted by the leaders.”
BITC workwell director Louise Aston urged business leaders to work together so that people didn't have to "suffer in silence".
"Organisations that do not promote the mental wellbeing of their employees risk long-term problems, including reduced competitiveness, lower productivity and fewer prospects for sustainable growth. Conversely, the rewards for businesses that engage with this issue are huge," she said.
