The gender pay gap is almost 15%, and widens "dramatically" after women have children, according to new research.
The Trade Union Congress said its study suggested that the average woman in paid employment effectively works for free for nearly two months of the year compared to the average man.
The gender pay gap means that working women must wait 54 days before they stop working for free, said the union organisation.
Women aged between 50 and 59 have the highest pay gap of 20.8%, while for those aged 60 and over it is 18.4%, according to the research.
The gender pay gap is largest in the South East of England (17.9%), East of England (17.5%) and the East Midlands (16.6%), said the TUC.
TUC general secretary Paul Nowak said: "Working women deserve equal pay, but at current rates of progress, it will take more than 20 years to close the gender pay gap.
"That's just not good enough. We can't consign yet another generation of women to pay inequality.
"It's clear that just publishing gender pay gaps isn't working. Companies must be required to publish action plans to explain what steps they'll take to close their pay gaps, and bosses who don't comply with the law should be fined.
"The pandemic highlighted that we can do more to help women balance their caring responsibilities and work. Flexible working is key to keeping mums in jobs and is our best way of closing the gender pay gap.
"We should change the law so that all jobs are advertised with all the possible flexible options clearly stated, and all workers must have the legal right to work flexibly from their first day in a job.
"It's clear that the gender pay gap widens dramatically once women become mums. We need ministers to fund childcare from the end of maternity leave to support working parents, along with better wages and recognition for childcare workers.
"Dads and partners need better rights to well-paid leave that they can take in their own right. Otherwise, mums will continue to take on the bulk of caring responsibilities and continue to take the financial hit."
The TUC said its study of official data showed that since 2011 the gender pay gap had fallen by an average of 0.4 percentage points a year, adding that at the current rate of progress, it will take until 2044 for pay parity to be achieved between men and women.
A Government spokesperson said: "The Government has taken significant action to support women at work. We have spent over £3.5 billion in the last three years to support families with the cost of childcare, and in December we announced that millions of employees will be able to request flexible working from day one of their employment.
"Wages are a decision for individual companies, but we strongly urge organisations to take steps to ensure female employees reach their full potential."
