British companies that do not report on their gender pay gap figures by Wednesday’s deadline should face “formal and public criticism” according to Sir Philip Hampton, who last year led the Government’s review into women in business.

Gender pay gap reporting rules come into force this week, requiring all UK companies with at least 250 staff to publish their wage gap figures between male and female workers.

The chairman of GlaxoSmithKline said that some of the numbers published so far had been “shocking” and it was questionable whether all firms had been legally compliant on equal pay, which is different to gender pay gap ­reporting and has been around for ­decades.

Around 80pc of the 9,000 eligible businesses had reported their pay gap numbers by Thursday evening, causing business leaders to urge firms to get a move on.

Some top law firms have reported figures that separate secretarial roles from partners so that the average gap looks lower.

“Sunlight is the best disinfectant,” Ms Morgan added. “Talent is going to be aware when companies are being cute with their numbers.”

The gaps exist because there are generally too few women in the most senior, best-paid roles.

While the number of all-male boards has dropped in recent years, there are still huge gaps in gender between very junior and senior jobs. In 2016, there were more men named David, Steve or Stephen leading a FTSE 100 firm than there were female bosses.