The gender pay gap has fallen to 5% for young people in their twenties but those gains are likely to be lost as women start to have children.
Research by the Resolution Foundation has found that the gender pay gap for millennials in their twenties has halved within a single generation but it says there is still work to be done as the gender pay gap then rises again as women enter their thirties and forties.
It means that millennial women will still earn significantly less than their male counterparts over their careers, according to the analysis by the Resolution Foundation.
Researchers have tracked the typical hourly pay of different generations of women over the course of their careers and compared it to that of their male counterparts. Although the gender pay gap has fallen to 5% for young people in their twenties, it rises to 9% by the time those same women reach the age of 30.
The Resolution Foundation says that, as men and women work for longer, tackling the gender pay gap at all stages of women's careers is key to reducing the earnings gap. Having children should not carry such a sharp and long-lasting pay penalty, it says.
"While many millennial women haven't experienced much of a pay gap yet, most probably will once they reach their thirties, when they start having children," said Laura Gardiner, senior policy analyst at the Resolution Foundation.
"What's more this pay penalty is big and long-lasting. Small hourly pay gaps quickly grow into large lifetime pay penalties that can leave women hundreds of thousands of pounds worse off over the course of their careers."
Commenting on the findings, Dr Carole Easton, chief executive of the Young Women's Trust, said: "Young women need action now to close the gap, or they face a lifetime of inequality."
Also this week, Brendan Barber, chair of Acas, said that the gender pay gap is likely to be one of the major employment issues in 2017 because of changes in the law. "To help reduce the gender pay gap in Britain's workplaces, large employers will have to report on differences between men and women's pay from April this year," he said. "Acas will be launching new advice for employers on best practice to help them comply with the new law."