More than 1,000 companies required to submit figures on the gender pay gap to the Government have not yet done so, just a day before the deadline.
Tomorrow (April 4) 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.
However by 10am this morning (April 3) figures show that just 7,590 have done so, with 79% of those having a pay gap.
Others either have no median gender pay gap (8%) or one in favour of women (13%).
Any companies which do not submit figures may face legal action including court orders and fines.
A spokesperson for the Home Office 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."
In the top 10 of those with the worst gender pay gap, the best-known company is Ryanair.
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 claims the disparity is because of the number of UK pilots it employs, 546 are male and only eight are female.
The gender pay gap is calculated as the difference between the average salaries of men and women. The national median gender pay gap is 18.4%.
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.