Even women who are just beginning their careers are paid less than their male colleagues in the U.K.
British women in their 20s are paid 5% less than men who are doing the same job, according to a new report by Resolution Foundation.
The think tank's research shows that women face a sizable pay gap even before they have children or spend time away from the office to start a family.
Robert Joyce, an associate director at the Institute of Fiscal Studies, said there are many possible explanations for the early-career pay gap, including discrimination.
"We can rule out one explanation though: young women are now actually more highly educated than young men, so the wage difference is not due to education," he said.
Related: Mothers still lag behind in pay for years
There is some good news: The gap is gradually getting smaller.
Women born between 1981 and 2000 have salaries that are 5% lower on average, while women born between 1966 and 1980 were paid 9% less than men when they were in their 20s.
Amanda Goodall, a senior lecturer at Cass Business School in London, said progress has been made because today's young women are better equipped to successfully negotiate salaries than previous generations.
The starter salary gap is also somewhat smaller than many women expect. A recent study found that female U.K. students expect to make 14% less than their male peers in their first jobs.
Related: Women do ask for pay raises, they just don't get them
The bad news? Today's young women can still expect to be underpaid throughout their careers.
"Small hourly pay gaps quickly grow into large lifetime pay penalties that can leave women ... worse off over the course of their careers," said Laura Gardiner of the Resolution Foundation.
Having children still carries a sizable pay penalty.
"Training, progression and promotion are much harder to come by when working part time, which many women with children either choose to do or feel they have to because of high childcare costs," Gardiner added.
