The proposals form part of a package of reforms that the party said would deliver a “step-change in how women are treated at work”.
Other measures announced by Dawn Butler, the shadow women and equalities secretary, include fining companies if they do not tackle their gender pay gap and forcing firms to publish a plan for supporting women going through the menopause.
Figures released last week revealed that the median gender pay gap among all employees currently stands at 17.3 per cent, falling to 8.9 per cent for full-time workers.
Labour said a government led by Jeremy Corbyn would create a new Workers’ Protection Agency and task it with reducing the pay gap. This would include fining companies that were not doing enough to close the gap, and lowering the threshold at which a company has to report the size of its pay gap from 250 employees to 50.
As part of new efforts to tackle sexual harassment at work, Labour would change the law to make employers responsible for dealing with harassment committed by third parties, including clients and customers.
Companies would also have to publish plans for tackling harassment and would be stopped from pre-emptively banning staff from speaking about discrimination or harassment.
Announcing the proposals, Ms Butler said: “Next Thursday, it is Equal Pay Day – the day when women effectively stop getting paid for the rest of the year compared to their male counterparts. It’s a disgrace.
“I’m sick of how women are treated at work. Audits aren’t enough, we know there’s a problem that needs fixing. So we will do something about it.”
Labour said it would extend statutory paid maternity leave from 39 weeks to 52 weeks and force employers to allow flexible working in all roles, unless they could prove that it was not suitable.
Ms Butler said: “We’ll also extend the amount of time a new mother can spend with her newborn by extending statutory maternity pay to 12 months, so that all mothers can afford to spend those vital early months with their baby.
“Labour will deliver a workplace revolution to bring about a step-change in how women are treated at work. We’ll boost pay, increase flexibility, and strengthen protections against harassment and discrimination.”
Under Labour’s plans, companies with more than 250 employees would have to publish a strategy for supporting women going through the menopause. The party said this should include flexible working policies, training for line managers and procedures for allowing menopausal women to take time off.
Labour pointed to a poll showing that a quarter of women going through the menopause considered leaving their jobs because of a lack of support from employers.
At the same time, the law would be changed to beef up the role of trade union equalities representatives. Currently equality reps do not have the same right to paid leave that other union officials are granted to allow them to carry out their duties.
Labour said it would change the law to ensure that equalities representatives have the right to paid leave, facilities and training.