More than 1,000 companies required to submit their gender pay gap figures to the British government have yet to do so, a day before the deadline.
By Wednesday an estimated 9,000 companies and public bodies with 250 employees or more must have submitted their median and mean gender pay gap data to the Government Equalities Office.
As of 10am on Tuesday, figures show that just 7,590 have done so, with 79% of those having a pay gap.
The rest of the employers either have no median gender pay gap (8%) or one in favour of women (13%).
Ryanair is the best-known company in the top 10 of those with the worst gender pay gap.
The airline pays women 71.8% less than men on average - when comparing median hourly rates, for every £1 men earn, women earn just 28p.
It says the disparity is because of the number of UK pilots it employs - 546 are male and only eight are female.
Companies who do not provide their figures could face legal action including court orders and fines.
A Home Office spokesman said: "Reporting gender pay gap data is not optional; it is the law, and employers that do not comply will risk facing legal action from the Equality and Human Rights Commission.
"This is the first year of reporting and we will use the results to target our efforts effectively as we continue to work with employers towards eliminating the gender pay gap."
The gender pay gap is calculated as the difference between the average salaries of men and women - it is not the same as equal pay, where firms are required to pay people doing the same job the same salary regardless of gender.
The national median gender pay gap is 18.4%.