Companies that miss the deadline could face legal action including court orders and fines, but only after being given more than a month’s grace period.
A spokeswoman for the Home Office said companies will still be able to publish their data using the Government website after the deadline, but added that their entries will be time-stamped.
The Equality and Human Rights Commission (EHRC), whose job it is to impose sanctions on non-complying firms, said it will write to employers that have not submitted their figures on April 9, giving them 28 days to do so “before an investigation takes place and an unlawful act notice is issued”.
Firms who publish their data late, but before April 9, will not face any enforcement action but will be kept on record.
Those who report their figures within 28 calendar days of the EHRC’s letter are expected to report on time next year to ensure no sanctions are applied.
Those who do not report their figures within the expected time frame will face investigation - if taken to court, an unlimited fine can be imposed.
Companies with 250 employees or more are required to publish their gender pay gap details under The Equality Act 2010 (Gender Pay Gap Information) Regulations 2017.
Chief executive of the EHRC Rebecca Hilsenrath said: “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, rethink their priorities, be prepared for serious reputational damage, and be ready to face a very unhappy workforce.”