Four out of 10 single women in their 30s do unpaid overtime at work, according to an analysis of official statistics by the Trades Union Congress (TUC).

Single women in every age group are more likely to do unpaid overtime than single men in the same age group, confounding the usual assumption that men put in the longest hours. Just over a quarter of men in their 30s do unpaid overtime.

Today’s figures show that while women who do not have children are more likely to do unpaid overtime than childless men, the gender gap reverses when they have children. The proportion of women doing unpaid overtime drops from 24.2% to 17% when they have children. However, the proportion of men doing unpaid overtime is relatively unchanged when they have children, dropping from 22.6% to 21.7%.

After several years of decline, the total value of unpaid overtime across Britain reached a record high of nearly £25 bn in 2007. The total number of employees working unpaid overtime increased by 103,000 to reach nearly five million. Employees who work unpaid overtime would receive an extra £4,955 a year if they were paid for the extra hours they are putting in. The average amount of unpaid overtime is seven hours and six minutes a week.

TUC general secretary Brendan Barber said: “Most people think long hours working is mainly a problem for men. But when you look at who is most likely to put in those extra unpaid hours at work the picture changes completely. Single women do more unpaid overtime than mothers, fathers and single men.

“Women who want to get on at work need to put in longer hours than anyone else, but as soon as they have children they no longer have that option. It is hardly surprising therefore that the senior levels of most organisations are male and that the gender pay gap stubbornly persists.”