Single women are more likely to do unpaid overtime than any other workers, a report has revealed.
On average, those in their thirties do 7.4 hours a week, equal to 43 days a year at their desks without being paid a penny.
It means millions of women are coming in early, skipping lunch breaks and leaving late.
Others find work intrudes into their weekends, with many having to reply to emails and take calls on Saturdays and Sundays.
Experts said many women feel they have to work harder to prove themselves in businesses where bosses are usually male.
Others have no choice, as huge workloads force them to put in longer hours.
The report, from the Trades Union Congress, says unpaid overtime peaks for single women between 30 and 39.
For many, this is the time in their career when they are trying to win a major promotion.
Others will have decided that they are no longer likely to start a family, so they will concentrate on work instead.
But Brendan Barber, TUC general secretary, warned that many bosses take such hard work for granted.
He said: "Those extra hours are too often taken for granted and, if allowed to build up, they can damage people's health, relationships and morale at work."
The report found that the arrival of children dramatically changes a woman's willingness to work overtime.
Just 17 per cent of working mothers do unpaid overtime - many simply cannot because they have to leave to collect their children from school or a child minder.
On average, working mothers out in 5.7 extra hours unpaid.
Overall, the report found that nearly five million workers regularly do unpaid overtime.
Teachers, lawyers, businessmen, architects and media staff are most likely to work overtime, paid or unpaid.
Women are working longer hours than ever - but men are doing the opposite, official figures show.
