Large fashion businesses, including the likes of Asos.com, John Lewis and Arcadia, will soon have to report differences in pay between male and female staff under a new law passed through parliament this week.
The House of Lords unanimously approved an amendment to the Small Business Bill without the need for a vote, which will force large firms in any sector with more than 250 employees to reveal average salaries of both genders, with a view to narrowing the current gap of around 20%.
Those that do not comply could face fines of a yet-to-be-decided amount, imposed by the Department for Business, Innovation & Skills (BIS).
The amendment is expected to be given royal assent by the Queen in the coming days, making way for the bill to become law within 12 months.
During this time the Secretary of State for BIS will consult businesses, trade bodies and auditors on how to implement and monitor the reporting.
Currently businesses can volunteer the information, but according to the Liberal Democrats only five companies have chosen to do so.
While delivering the final budget of the coalition government last week, Chancellor George Osborne said the gender pay gap is at its lowest level since records began, with men earning around 17.5% more than women on average per hour.