Do you have the guts to tell your boss you have depression or stress? The chances are that one in six of you reading this column will have a mental health condition at some time in your life. But I bet most of you believe that, if you were to admit to it, your career prospects would suffer. Or perhaps you think your manager will take the Janet Street-Porter view, that stress and depression are merely the “latest must-have accessory”.

When Labour leader Ed Miliband talked about tackling the “biggest unaddressed health challenge of our time” last month, he touched a nerve in the millions of people who have or have had a mental health issue. A quarter of the population will face a mental health illness in their life, one in six of the working population. Add to this the 1.5 million employees who are carers of people with mental illness and this will affect nearly every business at some stage.

But away from the celebrities who have “come out” about their depression, from Stephen Fry to Ruby Wax and Alastair Campbell, the stigma of mental health is such that most employees daren’t mention it. The result is that Britain’s employers and economy are losing out. According to the Centre for Mental Health, 91 million work days are lost to mental ill health every year. But these account for only half the £30bn cost of reduced productivity, with the rest taken up by employees turning up for work when unwell and not performing at their best.

There’s no single known cause for mental ill health, with current explanations suggesting a combination of brain chemistry, genetic and environmental factors. But early intervention from managers can help reduce the cost to business, as well as helping the employee. According to Nigel Wilson, group CEO of insurance provider Legal & General, intervening early enables the company to get three quarters of people back to work in a year.

“Specialist treatment like cognitive behavioural therapy can help people not just to come back to work but to stay at work in the first place,” he says.

With stress now the number one cause of long-term absence, and nearly one in three workers suffering anxiety or panic attacks due to work pressures, mental illness is an issue no employer can afford to ignore.