UK companies are rushing to submit their gender pay figures ahead of Wednesday’s midnight deadline.
By 11:00 BST, 9,053 companies had done so, with more than 1,000 companies reporting in the last day alone.
Firms with more than 250 staff must state the average difference between male and female employees.
Of those that have published data, 78% pay men more than women, 13% pay women more and 8% said they had no gender pay gap, based on the median measure.
One of the biggest names among the worst offenders so far is Ryanair, which reported a 71.8% gender pay gap.
Meanwhile, High Street brands KFC, Matalan, Starbucks, Costa, McDonald’s and Primark reported no difference in what they paid their female and male staff.
In terms of bonus gaps, several NHS trusts have some of the highest figures, with Hull and East Yorkshire Hospitals NHS Trust General Purposes Charity revealing a figure of 99.4%.
Some have criticised the exercise, which also applies to charities and public sector bodies, as a crude mechanism open to misinterpretation.
However, gender equality campaigning charity the Fawcett Society says it represents an opportunity for employees to talk about pay and find out what their colleagues earn.
Sam Smethers, the society’s chief executive, said gender pay gap reporting was a game-changer in terms of workplace culture and practices.
“Finally women are realising that they have a right to talk about pay and they cannot be silenced,” he said.
“By finding out what their colleagues earn, they are then in a position to challenge any pay inequality; it is much more common than people realise.”
CBI director-general Carolyn Fairbairn said gender pay gap reporting was a chance for companies for businesses to bring about change.
However, she warned that companies could not close the gap by themselves. “Many of the causes of the gender pay gap lie outside the workplace and will require a partnership between companies and government if we are to deliver long-term, lasting change.”
Ms Fairbairn said knowing the average pay difference between men and women would help companies develop more inclusive workplaces and support the rise of more women into senior roles.
It also found more than half of women would favour the company with the smallest pay gap or the one that was doing to most to close it.
Chloe Chambraud, gender equality director at Business in the Community, said bosses had to understand the factors driving their pay gaps and the groups of women who were most affected.
“Employers must address the root causes of inequality, from reducing bias and increasing transparency in recruitment, appraisal and promotion processes to normalising flexible working, for men and women, and offer financially viable parental leave packages,” she said.
“Only then we will ensure that men and women have equal lives at work and home.”