Mothers are being hit by a "pay penalty" if they work in part-time jobs, according to a new study.
Mothers tend to spend more time in part-time employment, so they do not benefit from pay rises associated with more experience, research found.
By the time a first child reaches the age of 20, mothers earn around 30% less on average than similarly educated fathers, said a report funded by the Joseph Rowntree Foundation and published by the Institute for Fiscal Studies.
Some of the gap is explained by mothers in part-time jobs or taking a break from work altogether, said the report.
The overall gender wage gap has fallen from 28% to 18% since the early 1990s for the less well educated, but has remained at 22% for the highest educated, the study reveals.
Even before they have children, women earn around 10% less than men, but the gap increases "rapidly" for many after they have children.
Monica Costa Dias, IFS associate director, said: "There are many likely reasons for persistent gaps in the wages of men and women which research is still investigating, but the fact that working part-time has a long-term depressing effect is an important contributing factor.
"It is remarkable that periods spent in part-time work lead to virtually no wage progression at all. It should be a priority for governments and others to understand the reasons for this.
"Addressing it would have the potential to narrow the gender wage gap significantly."
Robert Joyce, IFS associate director, added: "It is now the highest-educated women whose wages are the furthest behind their male counterparts, and this is particularly related to the fact that they lose out so badly from working part-time."
Sam Smethers, chief executive of the Fawcett Society, said: "What this study shows very clearly is that as a society we are not doing enough to value women's talents. That is a blow to our productivity and a huge problem for the economy as a whole.
"We need to make it possible for part-time work to keep women on the career ladder.
"Employers should offer all roles, including more senior ones, as flexible working unless there is a good business case not to, and create more senior part-time roles.
"It is time to change our jobs market to one which helps parents, especially mothers, to get on."
Helen Barnard, head of analysis at the Joseph Rowntree Foundation, said: "It's just not right that we treat part-time workers as if they are less valuable than full-timers.
"The majority of women working part-time are mothers, who often work part-time so that they can also take care of children or other adults.
"But they pay a heavy price for trying to balance these two roles. The poverty rate for part-time workers is double that for full-time workers.
"We can do something about this and redesign the jobs market so it works for everyone. Employers can make that happen by increasing the number and quality of jobs that are open to part-time workers - and hire flexibly rather than only allowing existing employees to negotiate part-time hours.
"In the meantime, as child poverty rises, the Chancellor should show he understands these pressures and ease the constraints facing low income part-time workers and their families by lifting the benefits freeze and fixing Universal Credit so families keep more of their earnings."
Dawn Butler, Shadow Minister for Women and Equalities, said: "This report reiterates the Government's consistent failure to adequately tackle the gender pay gap.
"This week, we will be celebrating the centenary of women's suffrage.
"Ministers must get serious, we can now mark 100 years since women were allowed to vote, but despite this milestone, women still face unacceptable pay disparities.
"The IFS report highlights the need to address the unacceptable fact that motherhood means some women take a hit to their earnings."
A Government spokesman said: "Action taken by this Government means that we are one of the first countries in the world to require all large employers to publish their gender pay gap and bonus data.
"This is not an option, it is the law.
"Through our modern Industrial Strategy we want to ensure the economy truly works for everyone which is why we committed to promoting fair and decent work for all and are working with businesses to make flexible working a reality for all employees across Britain."