The warning has been made by training company Pivotal Performance as the Health & Safety Executive (HSE) launches a new work-related stress website.
Its a huge problem with 442,000 people claiming work-related stress made them ill last year.
As the recession bites managers are putting more pressure on employees to perform, explained director Jane Gillham.
It can be any number of extreme pressures that can lead to stress. In general you should look out for unusual changes in behaviour. They might seem more tired than normal, eat and drink more or less, or not communicate as well as they usually do.
You may also notice they find it hard to make proper decisions or concentrate properly. And they may take more sick days. These can all be signs that you need to take some action to protect your team.
Pivotal Performance runs a number of training courses aimed at helping managers and directors correctly identify and tackle stress among employees.
In itself, stress is not an illness, but prolonged periods of it can cause mental or physical illness.
The new HSE website reveals that 13.6% of all people who work believe their job is very or extremely stressful.
Figures suggest for every 100,000 workers who worked in 2007 to 2008, 780 people first became aware of a work-related stress, depression or anxiety.
Jane added: Its good to challenge employees to be more productive, but managers need to be careful not to cause too much stress. It becomes a serious problem in the workplace when it affects the health of employees.
We welcome the new HSE website on work-related stress and hope it makes a difference to the way employers tackle this problem.
Pivotal Performance is one of the UKs leading health and safety training companies with offices in Northamptonshire and Scotland.
It has a unique way of training people called PPTplus, which teaches delegates how to win commitment from others, not just compliance.