More than half of UK companies failed to narrow their gender pay gaps in the second year of compulsory reporting.
Some 52% of firms did not reduce the pay discrepancy between men and women compared with last year, data shows.
The figures showed 78% of private companies had a pay gap in favour of men while 14% favoured women, with the remainder reporting no difference.
Firms had until midnight to file their gender pay gaps – by which time 10,428 had done so – or face legal action.
Across 45% of firms the pay gap widened in favour of men, while at a further 7% of companies there was no change.
Overall, the median pay gap in favour of men lowered slightly from 9.7% last year to 9.6% this year.
The median pay gap is calculated by comparing the difference in pay between the middle-ranking woman and middle-ranking man in the same companies.
By law, all companies, charities and public sector departments of 250 employees or more must publish their gender pay gap figures.
The Equality and Human Rights Commission (EHRC) has said it would take enforcement action against all firms that missed the deadline.
It was unclear exactly how many companies had not reported, although it is thought about a quarter did so in the last 36 hours before the deadline.
Among firms reporting the biggest increases in pay gaps were garage chain Kwik Fit, Interserve FS (part of the Interserve Group), and car retailer Inchape.
Firms reporting the biggest improvement in narrowing the gap were Newsquest Media Group, Mitie, and DFDS Logistics.
The three companies with the biggest gaps were Countrywide Services at 60.6%, which was nevertheless down from 63.4%; Independent Vetcare at 48.3% (50.5%); Easyjet at 47.9% (45.5%).
Want to find out the gender pay gap where you work? Try the calculator below.
A spokesman for Interserve Group said: “We are committed to addressing the issues on gender pay and through the leadership of our chief executive, Debbie White, we are making good progress.”
A spokeswoman for Kwik Fit said it was committed to narrowing the gap and both recruit more female staff and promote from within.
Easyjet said in March that the gap had widened from 45.5% as more female cabin crew had been recruited since the last pay snapshot.
Most of the airline’s pilots are male, and are more highly paid than cabin crew. The firm is making efforts to recruit more female pilots.
Ahead of the deadline, Peter Cheese, chief executive of the Chartered Institute of Personnel and Development, said it appeared that many companies still did not have an “action plan” of how they will narrow their pay gaps.
“Organisations that simply provide their numbers are failing to meet the increasing appetite and expectation for transparency amongst all stakeholders, including employees, investors, and regulators.
“Financial figures would never be given without any explanation for them, and gender pay gap reporting should be no different.”
He acknowledged, though, that a reason for the gap widening at some firms could be that they have initially focused on bringing more women into entry-levels roles in order to build a pipeline of female talent”.
TUC general secretary Frances O’Grady said: “Big employers clearly aren’t doing enough to tackle the root causes of pay inequality and working women are paying the price.
“Government needs to crank up the pressure.
“Companies shouldn’t just be made to publish their gender pay gaps, they should be legally required to explain how they’ll close them. And bosses who flout the law should be fined.
“We can’t allow another generation of women to spend their whole working lives waiting to be paid the same as men.”
Understanding the terminology
Median pay gap
The median pay gap is the difference in pay between the middle-ranking woman and the middle-ranking man.
If you place all the men and women working at a company into two lines in order of salary, the median pay gap will be the difference in salary between the woman in the middle of her line and the man in the middle of his.
Mean pay gap
The mean pay gap is the difference between a company’s total wage spend-per-woman and its total spend-per-man.
The number is calculated by taking the total wage bill for each and dividing it by the number of men and women employed by the organisation.
Pay gap v equal pay
The gender pay gap is not the same as unequal pay.
Unequal pay is giving women less than men for the same work. That has been against the law since the Equal Pay Act was introduced in 1970.
A company’s gender pay gap can also be caused by other things, for example fewer women in senior or highly-paid roles or more women in part-time jobs.
How does the BBC calculator work?
The individual company data reflects information submitted by companies to the Government Equalities Office.
The data submitted each year is based on figures drawn from a specific date – called the “snapshot date” – the previous year.
5 April is the snapshot date for businesses and charities. 31 March is the snapshot date for public sector organisations.
All gender pay gap figures in this article reflect the hourly median pay gap for all employees.
Calculator design and development: Irene de la Torre Arenas, Becky Rush, Scott Jarvis, Alexander Ivanov and Oliver Schnuck.
Data journalism: Clara Guibourg and Nassos Stylianou