Women working in equivalent full-time roles earn 22% less than men according to new research.
It means they are effectively unpaid for one hour and 40 minutes a day - a total of 57 working days every year. These are the findings of an annual survey of 72,000 UK managers published by the Chartered Management Institute (CMI) and salary specialist XpertHR.
The 2015 National Management Salary Survey has found that the pay gap has only slightly improved since 2014. For men and women of all ages and in all professional roles the gender pay gap now stands at £8,524, with men earning an average of £39,136 and women earning £30,612. In 2014, the pay gap stood at £9,069.
However, the pay gap rises to £14,943 for senior or director-level staff, with men earning an average of £138,699 compared to the average for women of £123,756. Women managers are also missing out when it comes to bonuses, with the average man's bonus almost twice that of the average woman's bonus.
The survey data also shows that the pay gap becomes wider as women grow older. Women aged 26-35 are paid 6% less than their male colleagues, rising to 20% for women aged 36-45. The gap increases to 35% for women aged 46-60.
Not only are women earning less, but there are also fewer of them in executive positions. Even though women comprise 67% of the workforce in entry-level roles, and continue to outnumber men in junior management roles, female representation drops to 43% at senior management levels. Just 29% of director-level posts are held by women.
The pay gap is largest in big firms, according to the report. New legislation coming into force in 2016 will require organisations with 250+ employees to report publicly on what they pay male and female staff.
Mark Crail, content director of XpertHR, said: "An entire generation has now worked its way through from school leaver to retirement since the first equal pay legislation came into effect in 1970, yet the gender pay gap persists, and many employers still prefer not to know just how bad it is in their organisation."
Ann Francke, CMI chief executive, said: "Transparency is a powerful driver for closing the gender pay gap. The Government's new reporting legislation is a welcome step forward and will be good news for business."
Peter Burgess, director at Retail Human Resources (which has an open salary policy) agrees: "It is clear more companies need to adopt open salary policies to help close the gender pay gap."
He added: "I say to managers, if you had someone who's earning a lower salary than someone else for the same job, could you look them in the eye and tell them why that's the case? [As] long as salaries are kept secret, ancient discrepancies in what people are paid are carried forward, even unwittingly, by employers."