Women without children are most likely to do unpaid overtime, says a report by the Trades Union Congress (TUC).
They are more likely to work extra hours than mothers, fathers and men without children, according to the union organisation.
However, it is men with children who put in the highest number of unpaid hours - 8.3 hours per week on average.
The TUC says five million workers in the UK do unpaid overtime, saving employers almost £25bn in 2007.
The report indicates that 24.2% of women without children do unpaid overtime - the highest number of any other group.
But this declines to 17% amongst women with children.
'Impossible choices'
The Fawcett Society, which campaigns for equal pay and conditions, said women were being presented with "impossible choices".
TOP JOBS FOR UNPAID HOURS
"They are forced to choose between caring for a family at home or maximising their career opportunities in a workplace that measures performance by the number of hours put in," the society's Kat Banyard said.
The TUC's general secretary, Brendan Barber, said this was preventing women getting the top jobs in their profession.
"It is hardly surprising that the senior levels of most organisations are male and that the gender pay gap stubbornly persists," he said.
Fathers' hours
Whereas women without children are more likely than men to work extra hours without pay, those men who do overtime do a higher number of hours unpaid.
And it is men with children who do the greatest number of unpaid hours, more than eight per week on average.
"Hourly workers who put in long shifts get paid overtime and often welcome the extra money in their pay packets"
"On average, fathers who put in unpaid overtime are doing almost an extra day a week. This can't be good for them, their children or wider society," Mr Barber said.
Teachers do the most unpaid work, an average of 11.2 hours per week. This is valued at more than £12,000 a year.
Workers in the legal, financial and media industries are also expected to work long hours without pay.
The TUC is urging employees to take proper breaks and says bosses should thank their staff by taking them to lunch.
The employers' organisation the CBI said it could be necessary for staff to work longer hours.
"Professional staff tend to work the longest hours and this is reflected in the more generous salaries they earn," a spokesperson said.
"Hourly workers who put in long shifts get paid overtime and often welcome the extra money in their pay packets," he said.
The TUC's research was carried out using official Office for National Statistics figures.
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