UK women earn on average 18 per cent less per hour than men, according to a major study of the gender pay gap.
The report from the Institute for Fiscal Studies, funded by the Joseph Rowntree Foundation, shows that while the average hourly wage gap has narrowed from 28 per cent in 1993, it has effectively remained the same among graduates for the past 20 years.
There is also a widening discrepancy between male and female wages after childbirth, with the gap growing consistently in the 12 years after a child is born to reach 33 per cent.
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Report co-author Robert Joyce said the overall narrowing was "the result of more women becoming highly educated, and a decline in the wage gap among the lowest-educated".
The wider disparity after childbirth was attributed to mothers missing out on promotions or accumulating less labour market experience, partly because many choose to return part time.
TUC general secretary Frances O'Grady told the BBC : "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."
Separate research commissioned by the Chartered Management Institute found men are 40 per cent more likely to be promoted into management roles than women, which it says is the key reason behind the stubborn gender pay gap.
Mark Crail, a content director at XpertHR, which carried out the study, said: "The gender pay gap is not primarily about men and women being paid differently for doing the same job.
"It's much more about men being present in greater numbers than women the higher up the organisation you go.
"Our research shows that this gap begins to open up at relatively junior levels and widens - primarily because men are more likely to be promoted."
