Women are effectively working for free for the rest of the year because of the gender pay gap, which will take 60 years to close at the current rate of progress, campaigners say.
The Fawcett Society called for more action from the Government and employers to tackle pay discrimination, job "segregation" and help women into senior posts.
Thursday has been labelled Equal Pay Day (EPD), with the society saying the 13.9% gender pay gap means women are effectively working for nothing from now until the end of the year.
Fawcett Society chief executive Sam Smethers said: "A root cause of the gender pay gap is that we don't value the work done by women.
"As we mark EPD this year we are focusing on the fundamental question of who and what we value and asking why it is that we don't value women and the work they do - paid or unpaid.
"Equal value goes to the heart of the fight for pay equality, because the reality is that if it is a sector dominated by women the pay will be lower.
"As we look ahead to a UK outside the EU and possibly the single market, we have to guard against the risk of going backwards and losing some of the rights that women have fought for over many years."
The society said in a report that the jobs women do are more likely to be low paid and they are less likely to receive a bonus or progress to the highest job in their organisation.
More than 50,000 women leave their jobs early every year after having a baby or becoming pregnant, said the report.
Men were urged to get involved in the campaign for pay parity, especially if they have jobs in areas traditionally dominated by women.
A separate report said one in five mothers have been overlooked for a pay rise or a bonus because they have had children.
A survey of 850 mothers by The Start Up Loans Company showed that more than a third believed they had been overlooked for promotion by spending more time at home than in the office.
Two-thirds said they thought they could have a higher paid job if they did not have children.
Another study, by totaljobs, found that men are twice as likely as women to feel comfortable asking for a pay rise.
A study of more than 4,700 employees and 145 employers showed that women typically expect to get paid a salary of £25,468, compared with £32,030 for men.
John Salt, director of totaljobs, said: "It is disheartening that our research has revealed that, despite efforts, gender pay equality remains a prominent issue.
"The application and interview process is a fantastic opportunity for both men and women to negotiate a fair benefits package, including a salary that meets their expectations.
"I would urge all female candidates to aim high and feel confident in demanding the same figure as their male counterparts."
Rebecca Hilsenrath, chief executive of the Equality and Human Rights Commission, said: "After over 40 years since the enactment of legislation banning sex discrimination at work and in pay, it is shameful that women continue to face barriers that hold them back. We simply cannot ignore the scale of the disadvantages that working women face.
"Girls and women outperform men at every stage in education, but time after time this success is not translated into rewards at work. Women are a vital part of the workforce and any proposals to tackle the gender pay gap must be strong enough to deliver the change everyone wants to see."
A Government spokesman said: "We now have the lowest gender pay gap on record and that's as a result of the changes we've made so men and women can share their parental leave, our work to get more women into the top jobs at our biggest companies and our drive to get more girls taking Stem subjects at school that will get them into more lucrative professions when they are older.
"But we know there's more to do. That's why we are requiring employers to publish their gender pay and gender bonus gap for the first time from April and we are giving working parents up to 30 hours of free childcare from next September, meaning women can go back to work and progress in their careers after having children if they choose to."