The average manager works the equivalent of 40 days a year unpaid because of the amount of overtime they have to put in simply to get their job done, according to research by the Chartered Management Institute (CMI).

The organisation's Quality of Working Life report, which has been issued in support of the TUC's Work Your Proper Hours campaign, also suggests managers are reluctant to take time off work even when they are sick because of the workload they have to contend with.

The CMI is warning businesses not to put too much pressure on managers, as it could adversely affect their performance and health, as well as making them more likely to look for an alternative job. This is even more of a danger in small companies, where individual managers have a vital role to play.

"Many organisations focus on the cost of absence to their organisations, yet are not addressing the root causes of absenteeism," said Jo Causon, director of marketing and corporate affairs at the CMI.

"Surely, in today's results-driven environment, output is more important than input, so two questions need to be answered: why are employers ignoring the impact of long hours on the health and performance of their employees and what responsibility are employees taking for how they manage themselves?"

According to the research, 89% of managers regularly work over their contracted hours, a figure that has hardly fallen since 2000 when it stood at 91%. The average manager works one hour and 18 minutes over contract every day, it added.

The majority (54%) of those who work excess hours do so because of the need to meet deadlines and the volume of work they face. Just one in three claims to put in overtime through choice.

But there are signs that a long-hours culture could be damaging to employee health and performance. Almost half (45%) said that the UK's long-hours culture affected their productivity and 40% thought it affected office morale.

On a personal level, 68% claim the amount of time they can devote to exercise - a key part of remaining productive - is affected by working over contracted hours, while 48% believes this prevents them from developing their personal skills.

"The perpetual cycle of taking out costs in recent years has meant that most organisations are driven to use their assets - particularly their people - more intensively," adds Causen.

"Yet it is clearly having a negative effect and will create longer-term problems for organisations unless the UK's long-hours culture is kept in check."