Nearly eight out of 10 companies and public sector bodies pay men more than women as the deadline passed for organisations to report their gender pay gaps.
Businesses with 250 employees or more were required to submit the data on mean and median gender pay gaps to the Government Equalities Office by midnight on Wednesday.
Companies who fail to provide data face legal action and there was a last-minute dash to file the information before the deadline - with more than 15% sending their information between Tuesday at 4pm and the cut-off point.
More than 10,000 companies submitted their data, with 78% of the 10,015 firms having a pay gap in favour of men.
The rest of the employers either have no median gender pay gap (8%) or one in favour of women (14%).
Ryanair is one of the best-known companies in the top 10 of those with the worst gender pay gap, along with the holding company for Millwall FC.
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.
Millwall Holdings PLC reported a median gender pay gap of 80%, compared to an average across all companies who have submitted data of 12%.
Meanwhile, a handful of employers have already filed figures for the 2018/19 financial year, including the Conservative Party’s campaign headquarters, which has reported a gender pay gap in favour of women of 15.7%.
The Tories did not publish a figure for 2017/18 as the party’s campaign HQ did not employ 250 or more employees. That threshold has now been passed, however.
Labour has reported a gender pay gap of 4.0% in favour of men, but this snapshot was taken on April 5 2017, and is not directly comparable with the Conservatives’ figure which is a snapshot as of April 5 2018.
Sam Smethers, chief executive of the Fawcett Society, said: “Gender pay gap reporting is a game changer in terms of workplace culture and practices. It forces employers to look at themselves and understand their organisations and it prompts employees to ask some hard questions.
“But even better than that, finally women are realising that they have a right to talk about pay and they cannot be silenced.
“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.”
Companies who do not provide their figures will face legal action including court orders and fines, but only after they have been given a month’s grace to report the figures.
The Equality and Human Rights Commission (EHRC) said it will write to employers who have not complied on April 9, giving them 28 days to publish the figures “before an investigation takes place and an unlawful act notice is issued”.
Courts can impose an unlimited fine on those who do not comply.
Of the largest employers in the UK - those with 20,000 people or more on their books - Lloyds Bank had the largest median pay gap at 42.7%, followed by the Royal Bank of Scotland (36.5%) and Lloyds Banking Group (32.8%).
At the other end of the scale, American Airlines had a gap in favour of women (-3.9%) as did BT (-2.3%), while Primark, McDonald’s and Costa were among those reporting no wage gap.
Of the Premier League football clubs, West Bromwich Albion has the highest median gender gap, of 34.3%, with Manchester City having the highest in favour of women, paying them 17.2% more than men on average.