The Institute for Fiscal Studies (IFS) found that while the overall ‘gender wage gap’ had narrowed over the past two decades, women with children were falling behind.
The study, carried out for the York-based Joseph Rowntree Foundation, found that on average, hourly pay rates for women are currently around 18 per cent lower than they are for men.
It compares to a gap of 23 per cent in 2003 and 28 per cent in 1993. However once women started a family, the gap widened consistently year by year.
By the time their first child had reached the age of 12, their hourly pay was 33 per cent less than men’s hourly pay.
The report suggests the difference may be due to women working fewer hours once they have children and, as a consequence, miss out on promotions – or simply accumulate less labour market experience – while their male colleagues pull further ahead.
It also found the closing of the overall wage gap was down to improvements in the pay rates of less well-qualified women, who did not have A-levels or higher qualifications, while for better-educated women the gap had remained unchanged for 20 years.
IFS director Robert Joyce 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.”
TUC general secretary Frances O’Grady said: “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.
“Without more well-paid, part-time jobs and affordable childcare, the gender pay gap will take decades to close.
“We need to see a step change in government policy and employer attitudes if we are to fix this problem.”
A Government spokesperson said: “The gender pay gap is the lowest on record but we know we need to make more progress and faster.”
Labour’s shadow women and equalities minister, Angela Rayner, said: “There is no excuse for this – women deserve equal pay for equal work.”
She added: “Mothers should not be penalised for having a family. I expect a government led by a female prime minister to stamp out such wage discrimination.”
The news comes as another study of 60,000 people revealed that men are more likely to have been promoted to a senior management job in the past year.
Research from the Chartered Management Institute and XpertHR also showed that the average salary for a male manager is almost £40,000 – around £9,000 more than for women – and that men received higher bonuses.