Women who have children earn up to 45 per cent less than those who do not in what experts have called a "motherhood penalty".

Women’s earnings were on average 28 per cent, or £306 a month, in the first year after the birth of their first child of what they would have earned if they had remained childless, according to the UCL research.

This "motherhood penalty" rose to 45 per cent when the next six years were taken into account according to the study, published in the European Sociological Review.

Older mothers aged over 30 and those who were highly educated suffered a smaller penalty, potentially because they had career-oriented jobs to which they could return.

The researchers speculated that this could be because they had established a career with an employer who valued their progression in a company when they returned to work.

A key determinant in the size of the "motherhood penalty" was the amount of hours that women worked after the birth of their children. Those who returned part-time suffered a bigger reduction in earnings than those who were able to work full time.

"Working hours was one factor that did stand out in explaining the earning penalty," said Dr Giacomo Vagni, a researcher at the centre for time user research at UCL’s Institute of Education.

"We found that mothers who are able to maintain full-time working hours experience little to no penalty. However, few mothers manage to do so. After the birth of their first child, few manage to return to their pre-birth working hours."

The study used the British Household Panel Survey to compare childless women with those who had children between 1995 and 2005.

It found that childbirth in a family had a "small negative" effect on overall household income, which would suggest the fathers either took on new jobs or increased their hours. There was, however, variation around that average, with some experiencing significant reductions.

By contrast, highly-educated households saw a growth in household income after the first birth compared to other families, possibly because they were able to access childcare to return to work quickly, supported by their employers.

"One of the most important mechanisms through which motherhood affects a woman’s career is that the birth of a child creates a very difficult ‘work-care balance’ for mothers," Dr Vagni said.

"The UK is a country with a very expensive private childcare system. Mothers are much more likely to be the ones putting their career aside with the birth of a child.

"Therefore, motherhood takes mothers away from the labour market during the prime years of career development. Mothers with young children miss out on important job opportunities and promotions.

"Often, when mothers return to the labour market, they do so on a part-time basis. We know now that part-time work does not contribute to meaningful career progression." 

Dr Vagni said employers "also take advantage of the fact that mothers are looking for flexible working arrangements and might offer them lower wages in return" and fathers "do not face this trade-off because they generally manage to take the back seat and enjoy the ride of parenthood".   He said the "motherhood penalty" accounted for a significant part of the overall gender pay gap, which could only be closed with better subsidised childcare and child-friendly policies by employers.