The gender wage gap is smaller when comparing young women – before they become mothers – with their male peers.

But the gap widens consistently for 12 years after the first child is born, by which point women receive 33% less pay per hour than men.

The widening of the hourly wage gap after childbirth is down to reduced hours of work, not because women see an immediate cut in hourly pay when they reduce their hours, according to the report.

“Rather, women who work half-time lose out on subsequent wage progression, meaning that the hourly wages of men, and of women in full-time work, pull further and further ahead. In addition, women who take time out of paid work altogether and then return to the labour market miss out on wage growth,” it said.

The research also found the gap in average hourly wages between male and female employees has been falling over the past two decades.

The gap is now 18% compared with 28% in 1993 and 23% in 2003.

However, for graduates or people with A-levels, the gender wage gap is essentially the same as it was 20 years ago.

Robert Joyce, associate director at IFS and an author of the report, said: “Women in jobs involving fewer hours of work have particularly low hourly wages, and this is because of poor pay progression, not because they take an immediate pay cut when switching away from full-time work. Understanding that lack of progression is going to be crucial to making progress in reducing the gender wage gap.”