The gender pay gap more than trebles when women reach their 30s, according to a report published by the Trades Union Congress.
The report said adult women in all age groups earn less than men of the same age but the sharpest increase in the gender pay gap occurs when women reach their 30s.
The difference between men's and women's full-time earnings rises from 3.3% for women aged 22-29 to 11.2% for women aged 30-39.
The ‘motherhood penalty’ partly explains why the gender pay gap increases so rapidly for women in their 30s, the report said.
TUC general secretary Brendan Barber said: ‘Despite girls outperforming boys at school and at university, too many employers are still failing to make use of women's skills. When women earn poverty wages, the whole family suffers.’
But minister for women Harriet Harman said: ‘The gender pay gap has fallen from 17% to 12% in the last ten years, and there will be some tough measures in the new equality bill which will come out later this year, to cut it even further.’