The biggest gender pay gap hits women in their 50s, who earn almost a fifth less than men of the same age, according to new research.
A study by the TUC found that full-time women workers in their 50s earned an average of just under £12 an hour, compared with £14.69 for men.
Women in their 50s also earn less than women in their 30s, according to the report, published ahead of new pay and unemployment figures.
The TUC said that half of women in their 50s were in part-time jobs, with most earning less than £10,000 a year.
There has been a huge increase in the number of women working past the age of 50, but the TUC voiced concern about the quality of jobs they were offered.
TUC general secretary Frances O'Grady said: "The rising number of women over 50 in work has been the great success story of the last two decades.
"But while more women are working for longer, low pay remains a big problem, particularly for the millions who have been badly let down by the pension system and who are approaching retirement with little or no savings.
"Women over 50 earn nearly a fifth less than men of the same age and the pay divide for the 1.6 million older women with part-time jobs is even starker.
"Part-time work is the only option available for many older women who also have to look after loved ones, either their grandchildren or their own parents, or who may no longer be able to work long hours.
"As the workforce ages, it's essential that people are able to work part-time hours without having to give up decent pay. Most of us will have caring responsibilities at some point in our lives and it's not right that looking after loved ones still has such a damaging effect on women's pay and career prospects."