Video report by ITV News Political Correspondent Emily Morgan
Firms across the UK have just a few hours left to submit their gap in pay between men and women.
The findings will soon expose which companies have the biggest discrepancies when it comes to salary.
Of all the firms to have submitted their details so far, 78% are shown to have a gap - with airline Ryanair paying women 71.8% less than men on average.
You can find out the gender pay gap at your workplace here.
So do all companies have to submit their figures?
Only companies with 250 employees or more have to submit their median and mean pay gap data.
This equates to roughly 9,000 businesses and public bodies across the UK.
The data will show the average gap in pay between men and women from a range of roles across a company, as opposed to the difference between a man and woman doing the same job.
When is the exact deadline then?
The clock runs out at midnight on Wednesday.
By then, companies must have submitted their figures to the Government Equalities Office.
Those who fail to comply will be written to by the Equality and Human Rights Commission on April 9, giving them 28 days to publish the figures before an investigation is carried out.
This could result in a court-imposed unlimited fine.
Who are the winners and losers so far?
The national median gender pay gap currently stands at 18.4%.
Amongst local authorities North Hertfordshire district council reported the highest gap of 34.0%, which means on average women earn 66p for every £1 that men earn.
By contrast, Adur district council reported a gap of 50.3% in favour of women.
The universities with the biggest gaps were Harper Adams University and York St John University who reported a gap of 37.4%. This means on average women earn 63p for every £1 that men earn.
Three institutions have no gender pay gap at all, the Royal College of Art, the Royal Agricultural University and the Royal College of Music.
Every police force in England and Wales has published their gender pay gap. Derbyshire Police reported the highest figure of 28.8%. This means on average women earn 71p for every £1 that men earn.
Cleveland Police had no gender pay gap. No force reported a figure in favour of women.
The highest gender pay gap among the UK's main newspapers is 23.4%, reported by the Telegraph. The smallest is 12.1%, reported by the Guardian.
What are the positive stories?
One which hasn't recorded issues with a gender pay gap is IT consultant firm FDM Group.
FDM managed to achieve a 0% gender pay gap, despite the fact that 75% of its staff are male.
Lara Plaxton, head of HR at the firm, said that the company had worked hard over 10 years to eliminate the gap, employing a range of initiatives including recognising female talent in the tech world.
But Ms Plaxton said the main change had been brought about by a "shift of culture" at leadership level.
Tell me more about Ryanair
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.
Ryanair says the disparity is because of the number of UK pilots it employs - 546 are male and only eight are female.
What would happen afterwards?
Companies that do not comply with the request could face investigative action afterwards, possibly resulting in a fine.
Several companies, including ITN, have already pledged to eliminate their respective gender pay gaps.
Theresa May also has vowed to tackle the "burning injustice" of the gender pay gap.
In an article for The Telegraph, the Prime Minister compared the pay gap to the Suffragette campaign a century ago - adding that "major injustices still hold too many women back".
She wrote that progress is still too slow and action is needed to close the gap.
"It is essential that we do so. Most importantly, because equality for women is a right, and our whole society is the poorer as long as it remains unrealised," Mrs May said.
"There is also a clear economic imperative. It is estimated that if women and men enjoyed parity in their hours, pay and seniority at work then we could see up to £150 billion added to our GDP."