Well-educated mums who choose to work part-time still suffer a "pay penalty" over time, a new report by independent U.K. economic research unit the Institute for Fiscal Studies (IFS) has found.
Calls have been made for power-holders to "redesign the jobs market" after the IFS study shows that the gender pay gap accelerates even more rapidly for mothers who opt for part-time jobs.
Overall, the U.K. gender wage gap has fallen from 28 percent to 18 percent since the early 1990s for the less well-educated, but has remained stuck at 22 percent for the most educated, the study reveals.
Mums tend to spend more time in part-time employment, and therefore don't reap the pay rises associated with more experience, research found.
By the time a first child reaches the age of 20, mothers earn around 30 percent less on average than similarly educated fathers, said the report, funded by social policy research and development charity the Joseph Rowntree Foundation.
The gap is partly explained by mothers who work in part-time jobs or take a break from work altogether, said the report.
Even before they have children, women earn around 10 percent less than men, but the gap widens 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 gender-equality campaigners 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 work – 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 that 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 [U.K.] 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, U.K. shadow minister for women and equality, 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."