Companies have been warned that they will face court action and potentially unlimited fines if they fail to comply with gender pay gap reporting requirements.

The warning comes from the UK’s Equality and Human Rights Commission just ten days from the 4 April deadline for businesses and charities. Public sector bodies face a cut-off date of Friday this week (30 March).

Reports estimate there may be as many as 5,000 companies still to comply with the requirements, which ask companies with more than 250 employees to reveal their gender pay gaps in published reports.

Rebecca Hilsenrath, chief executive of the Equality and Human Rights Commission, said: “Employers with 250 or more staff still have time to report their gender pay gap. The clock is ticking and with just 10 days to go, those who haven’t reported really are entering the last chance saloon.

“This is not optional; it is the law and we will be fully enforcing against all companies that do not report.

“This legislation is in place to bring about better gender equality in the workplace and any employer not complying needs to ask themselves tough questions, re-think their priorities, be prepared for serious reputational damage, and be ready to face a very unhappy workforce.”

Once the deadline passes, the commission will send letters to companies on 9 April, giving them 28 days to comply with the law before an investigation takes place and an “unlawful act notice” is issued.

Jemima Olchawski, head of policy and insight at the Fawcett Society, a charity that campaigns for gender equality, said: “The gender pay gap represents a productivity gap. It’s bad for women who lose out on potential earnings and career opportunities but also bad for businesses who are failing to properly recruit, promote and reward women.

“Pay gap reporting is an opportunity to look at the data, understand the cause and nature of the gap in an organisation and develop a plan to close it.

“McKinsey estimate that the UK could add £150bn to GDP if we improve performance on gender balance in the workplace—so employers really can’t afford not to act.”