Leaving the EU would turn the clock back on women's rights by "decades", hitting equal pay and protection against pregnancy discrimination, a new report has warned.
The TUC said the EU had been instrumental in empowering working women and helping them challenge inequality.
The union organisation said "huge" gains had been made as a result of membership, including equal wages for hundreds of thousands of low-paid women, and paid holidays for 1.5 million part-timers.
TUC general secretary Frances O'Grady said: "Women have made huge gains in the workplace as a result of EU membership, ranging from protection against pregnancy discrimination to fairer pay, holiday and pensions.
"Brexit risks turning the clock back decades on these hard-won rights.
"I think we should all be very worried when he hear leading Brexiters describing EU social and employment protections as burdens. These laws have helped to improve the lives of millions of working women.
"If we pull out of Europe all the leading employment law experts agree that it will be worse for workers' rights, and it is women who stand to lose most."
Vote Leave chairwoman Gisela Stuart said: "The UK has led the way in promoting and protecting workers' rights, and it's deeply misleading to suggest that leaving the EU would put them at risk.
"The best way to ensure that workers' rights are protected is to have a parliament that is directly accountable to the people; in other words, to put power back in people's hands.
"The EU prevents that, and that means that unelected and unaccountable bureaucrats can impose rules and regulations that the British people have no say over."
But fellow Labour MP Yvette Cooper, speaking for Britain Stronger in Europe, said: "The EU has been a powerful force for women's rights over the past few decades - giving part-time workers equal rights in the workplace, ensuring access to maternity pay and making sure women are able to attend doctors' appointments whilst they're pregnant.
"Leave campaigners claim that British women have nothing to worry about when it comes to Brexit but the truth is that many of them have campaigned vigorously and doggedly against legislation that has helped put women on an even kilter with men."