Women face a widening pay gap with their male counterparts once they start a family, a study by a leading economic think tank has found.
The Institute for Fiscal Studies (IFS) found that while the overall "gender wage gap" had narrowed over the past two decades, women with children were falling behind.
The study, carried out for the Joseph Rowntree Foundation, found that on average, hourly pay rates for women are currently around 18% lower than for men. That compares with a gap of 23% in 2003 and 28% in 1993.
However once women started a family, gap widened consistently year by year so that by the time their first child had reached the age of 12, their hourly pay was 33% down on men.
The report suggested the difference may be down to women working fewer hours once they have children and, as a consequence, missing out on promotions - or simply accumulating less labour market experience - while their male colleagues pull further and further ahead.
It also found the closing of the overall wage gap was down to improvements in the pay rates of less well-qualified women, who didn't have A-levels or other higher qualifications, while for better-educated women the gap had remained unchanged for 20 years.
IFS director Robert Joyce, a co-author of the report, said: "Women in jobs involving fewer hours of work have particularly low hourly wages, and this is because of poor pay progression, not because they take an immediate pay cut when switching away from full-time work.
"Understanding that lack of progression is going to be crucial to making progress in reducing the gender wage gap."
TUC general secretary Frances O'Grady said: "It is scandalous that millions of women still suffer a motherhood pay penalty.
"Many are forced to leave better-paid jobs due to the pressure of caring responsibilities and the lack of flexible working.
"Without more well-paid, part-time jobs and affordable childcare, the gender pay gap will take decades to close. We need to see a step change in government policy and employer attitudes if we are to fix this problem."
A Government spokesman said: "We want to make our country a place where there is no limit on anyone's ambition or what they can achieve - that means making sure everyone, regardless of their gender, can succeed at work.
"The gender pay gap is the lowest on record but we know we need to make more progress and faster. That's why we are pushing ahead with plans to force businesses to publish their gender pay and gender bonus gap - shining a light on the barriers preventing women from reaching the top.
Labour's shadow women and equalities minister, Angela Rayner, said: "It is unacceptable that the wage gap between men and women with 'A' levels or degrees has remained unchanged over the last 20 years.
"There is no excuse for this - women deserve equal pay for equal work. There is little incentive for those young women who have just qualified with their 'A' levels and are considering university, to see that in the future they will still be paid less than the men they study alongside."
She added: "I expect a government led by a female prime minister to stamp out such wage discrimination."