Four in ten employers say some of their workers have agreed to work more than 48 hours a week, going beyond the limit set in the EU Working Time Directive.
The directive, introduced in October 1998, says the maximum working week for most employees should not exceed an average of 48 hours unless the employee has signed an opt out.
A study in the IRS Employment Review showed 42 per cent of staff had agreed to work more than 48 hours.
The directive is being reviewed by the European Commission and the study showed employers were keen to retain the UK opt-out right.
The study found the typical week was 38 hours for manual and 37 hours for white-collar workers.
Manual workers, including production workers, cooks, caretakers, cleaners and security guards, were most likely to work unsociable hours.
Non-manual workers working out of normal office hours included call centre staff, journalists, police and social workers.
Many organisations have adopted flexible working practices.
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