All companies and charities with more than 250 employees are now legally required to publish those details on their own websites and a government site dedicated to the topic.
Many failed to do so by an extended deadline that expired this week, the Equality and Human Rights Commission said in a statement.
"Breach of these regulations is breaking the law and we've always been clear we will enforce with zero tolerance," said its chief executive Rebecca Hilsenrath in a statement.
"Last month, we contacted almost 1,500 businesses to commence enforcement proceedings and as a result the number of employers facing investigation is now under 500."
The Commission did not give an exact figure, but said it was in the hundreds.
As in many other countries, gender pay inequality has been a persistent problem in Britain despite sex discrimination being outlawed in the 1970s, and has sparked a public debate in recent years over why wages are still so different for men and women.
A law introduced last year requires companies and charities with more than 250 workers - covering almost half of Britain's workforce - to report their gender pay gap each year by April 4.
HSBC and Virgin Atlantic had the biggest gender pay difference of companies in the UK with over 5,000 employees, at 59 and 58 percent respectively, according to a Reuters data analysis which used the mean as the measure.
Around 11,000 employers had published their pay details by the extended deadline, which expired on Monday.
Companies are not required to break down the data in detail, leading to criticism that the average figures could obscure or exaggerate demographic explanations for disparities. Yet they mark a turning point for women in the workplace, advocates say.
The Fawcett Society, which lobbies for gender equality, said last month that gender pay gap reporting was "a game changer in terms of workplace culture and practices".
But it warned that enforcement was a "disproportionately drawn out process", calling for measures to introduce swifter penalties for companies that failed to comply.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)
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